Source: web.mit.edu
Most of the time, a trump opening lead should be rejected. One primary reason is that frequently it will remove a guess for declarer or outright blow a trick. In all the below examples, hearts are trumps and the lowest card has been led:
 AJ85 |
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| 763 |
Q2 |
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| KT94 |
The percentage play for declarer is to take the finesse after cashing a high honor. If you lead a trump, declarer can’t ever go wrong.
| AT64 |
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| K83 |
J5 |
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| Q972 |
The defense will always get a trump if it waits for declarer to start the suit. A low heart lead here, and declarer beats the J with his Q and then finesses the T for no losers. Notice that East might duck the heart (3, 4, 5, 7) and make declarer guess what to do, but the subsequent lead of the heart Q will play the suit for no losers.
| AT64 |
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| J82 |
Q5 |
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| K973 |
Same story as above – the heart lead blows a natural trump trick.
| QT53 |
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| A4 |
J97 |
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| K862 |
As declarer, the percentage play in this trump suit (assuming entries exist to both hands in the side suits) is low to the K, and if that wins, low to the T. Today declarer would have lost two tricks, if not for the fact that West led the heart ace and continued hearts, holding declarer’s losers to 1.
The other main pitfall of a trump lead is that it gives up a vital tempo. The opponents have the auction:
| South | West | North | East |
| 1 |
pass | 2 |
pass |
| 2 |
pass | 4 |
pass |
| pass | pass |
West holds: 
K972 54 
K972 K63
It is true that leading from a king has the potential to blow a trick. With no basis favoring the lead of either pointed suit, West tried a trump. Unfortunately, the whole hand was this:
East won the ace of hearts, and tracked back a diamond, but it was already too late. South rose with the ace of diamonds, drew a second round of trumps, and successfully ran the club 9 pitching away all his losers. In a matchpoint game, this lost trick will almost always matter greatly. In retrospect, West should see that the club king is a likely liability with clubs bid on the left, and make an attacking pointed suit lead. Even when you blow a trick, it might not matter as that lost trick might have been going away on the run of the clubs anyway.
So, when should we lead a trump?
1. Partner has left in a takeout double at a low level
K973 AQ62 KJ74 8
| South | West | North | East |
| 1 |
double | pass | pass |
| pass |
What just happened? Well, unless partner has taken leave of her senses, she should have really good clubs to leave this in at the one level. The immediate goal is to prevent ruffs in dummy; get that club 8 on the table.
2. We’ve doubled a part score on power when game is in doubt
973 Q962 A87 KT6
| South | West | North | East |
| 1NT (15-17) | |||
| 2* |
double** | pass | 2 |
| 2 |
double | pass | pass |
| pass |
Alert 1: *– a single suited hand (Cappaletti)
Alert 2: **– Double = stayman (systems ON over artificial x and 2; systems OFF above that)
Partner opens a 15-17 NT, and we have 9 HCP planning to invite. RHO comes in anyway, and we double to show them we won’t be pushed around when we have 24-26 HCP! Again, the biggest disaster would be a ruff in dummy, and it is possible declarer can’t get to dummy to lead toward his winners. Play a trump and watch for partner’s signals! Doubled part scores are bottoms when they make.
3. When we’re going passive, and we have evidence a trump lead will not blow a trick
KJ73 62 KJ74 AQ8
| South | West | North | East |
| 1 |
double | 2NT* | pass |
| 4 |
pass | pass | pass |
* = Jordan 2NT. It is a four card limit raise or better in hearts. Remember, when we are responder we choose redouble with 10+ points and no fit for partner, and 2NT when we have a fit. Redoubling with a fit is a mistake I see frequently with newer players. It allows East to get in a cheap bid to tell West what to lead.
Because the opponents have shown a 9 card fit in hearts, the trump lead is much less likely to blow a trick than in the earlier examples. In addition, they’ve bid game despite our 14 HCP behind the opening bidder so partner rates to have nothing. We may not beat the contract, but our goal is to try to give up no extra tricks.
4. When you have a strong holding in declarer’s first suit
AQ973 73 A62 852
| South | West | North | East |
| 1 |
Pass | 1NT* | Pass |
| 2 |
Pass | 3 |
Pass |
| 4 |
pass | pass | pass |
*1NT = forcing, they play 2/1 You see your RHO reach for the bidding box, and you are all set to overcall one spade, when that is what he grabs. Try to pass in tempo; don’t give away your holding by pausing uncomfortably or asking your opponent if they meant to bid 1S! They limp their way into game in hearts, and the heart lead is definitely called for. You don’t want too many of your natural spade tricks going away on ruffs in dummy. Lead a heart, and when you get in again with one of your entries, lead a second heart. The cards lie well for us and we should beat this; it might even be a good time to risk a double because things presumably lie so well for us, but that would be aggressive with no trump tricks.