This study tested whether synthesized human male pheromones increase the sociosexual behavior of men. Thirty-eight heterosexual men, ages 26-42, completed a 2-week baseline period and 6-week placebo-controlled, double-blind trial testing a pheromone "designed to improve the romance in their lives." Each subject kept daily behavioral records for 6 sociosexual behaviors: petting/affection/kissing, formal dates, informal dates, sleeping next to a romantic partner, sexual intercourse, and self-stimulation to ejaculation (masturbation) and FAXed them each week. Significantly more pheromone than placebo users increased above baseline in sexual intercourse and sleeping with a romantic partner. There was a tendency for more pheromone than placebo users to increase above baseline in petting/affection/kissing, and informal dates, but not in self-stimulation to ejaculation or in formal dates. A significantly larger proportion of pheromone than placebo users increased in > or = 2 and > or = 3 of the 5 sociosexual behaviors involving a female partner. Thus, there was a significant increase in male sociosexual behaviors in which a woman's sexual interest and cooperation plays a role but not in male masturbation which involves only the man. These initial data need replication but suggest that human male pheromones affected the sexual attractiveness of men to women.