Leks, communal display grounds in which males display and females only attend to mate, represent one of the most remarkable outcomes of sexual selection. There have been no detailed studies that compare the behavior of males and females between leks of different sizes to test if there is any benefit for male clumping and to test the many hypotheses suggested to explain lekking. In the black grouse, Tetrao tetrix, larger leks have many more female visits and copulations, leading to a higher average male mating success. Females visiting larger leks are also more likely to mate, indicating that female preferences of males are important for the evolution of leks. Yearling males seldom copulate, but their presence on the lek increases the mating success of adult males, suggesting that lek size per se and not only male quality affects female preferences for larger leks. The distribution of males over lek sizes agrees with a game theory model of ideal free distribution with unequal competitors, in which less competitive males go to smaller leks.