On the opening lead, it is usually wrong to lead an Ace when you do not own the King in that suit. Here are the exceptions—times when you SHOULD lead an Ace without a King—or should at least consider it.
- The opponents have bid to 5 of a minor suit, by passing 3NT. Their bidding should tell you the suit in which they lack a stopper. Lead the Ace even if you don't have the King. Partner should have the King. (If either opponent had King, they'd have tried for nine tricks in NT instead of eleven in a minor.)
- Against a small slam in a suit, lots of people bid “bad Blackwood” asking for Aces with a worthless doubleton in a side suit. If you have an Ace, lead it—unless the Declarer on your right has cue-bid the King. If Declarer has cue-bid King, wait for him/her to come to you. Sometimes your partner has the King. Do NOT lead an Ace without a King against a no trump slam when it is a “power auction” as Aces are made to kill Kings and Queens (to promote lower-ranking honors in your hand or partner's hand—hopefully). However, if you know they are bidding NT based on a long suit, it might be right to lead the Ace—you might not get it otherwise.
- When opponents cue bid, looking for slam, and stop at the 5 level. You should know the suit in which they lack controls. Lead the Ace. Your partner has the King.
- When your partner makes a lead-directing double and you have Ace in that suit. Lead the Ace.
- At the 5 level and above, if you feel you are not getting many tricks, go ahead and lead an Ace without a King. This is particularly advisable if Dummy has indicated a good side suit on which Declarer may discard all his/her losers.
- Against a Gambling 3NT, lead an Ace to look at Dummy (and try to find five tricks before Declarer gets his/her eight tricks in a running minor plus something else).
- If you have Ace-doubleton in a suit partner has opened or bid and have no other appealing lead, lead the Ace and hope partner has King and can give you a ruff.
- If you have Ace-doubleton in a major partner has bid, and the opponents are in no trump, go ahead and lead the Ace and hope to get out of partner's way (unless you have another very strong lead and prefer to keep the Ace as an entry for your suit (and perhaps an unpleasant surprise for Declarer who is expecting partner to have that Ace). This lead is much less appealing when partner opens a minor.
- When you can tell from the auction (and your holding in a suit) that your partner is likely to be short, lead the Ace and hope to give partner a ruff on the next round. (E.g., 1♠ on your right, pass by you; 2♣ on your left; pass by partner; 3♣ on your right; pass by you; 4♠ on your left. You hold Axxx in clubs. There is a good chance that partner has a singleton club. Lead the Ace and another club. If you have another quick entry, the club spot you play second round tells partner what suit to play back to you to get another ruff.)