Two experiments were conducted to evaluate the ability of boars to identify estrous sows. The preference of 11 mature boars for estrous or non-estrous sows was tested using a T-maze in Experiment 1. Boars were given a feed test on Days 1 and 4 and on Days 2, 3, 5 and 6; estrous and nonestrous sows were alternated in the right and left arms of the T-maze. The time (min) boars spent near four sets of estrous and non-estrous sows was recorded during each 5 min test. Yorkshire, Duroc, Landrace and Hampshire boars were tested in a random order on each day. Three boars (called target boars) spent more time (P<0.05) near estrous rather than non-estrous sows. No boar preferred non-estrous sows. In Experiment 2, the time (min) that 8 crossbred sows (Yorkshire, Duroc, Landrace and Hampshire crosses) spent near target boars was determined using a 5-min T-maze preference test. None of the 8 estrous sows tested showed a preference for any of the 3 target boars (P>0.50). We conclude that some individual boars have the ability to distinguish between estrous and non-estrous sows. The target boars' ability to detect an estrous sow is not determined by feedback from the behavior of estrous sows.