Women's participation in clinical trials, particularly those involving drugs, has been said to be both overrepresented and underrepresented. How can this be? Studies of participation are compared and contrasted to elucidate some reasons for this contradiction. The history of women's participation in clinical trials is chronicled through policies and regulations filled with restrictions. Since 1993, however, the National Institutes of Health has mandated, and the Food and Drug Administration has emphasized, inclusion of women in clinical trials, only to be thwarted by other regulations excluding many women. Gender-specific analyses are required to detect gender differences in effects of pharmaceutical and nonpharmaceutical interventions, but they are seldom performed. The exclusion of women from clinical trials means that women's healthcare is compromised by lack of sex-specific information about dosing of drugs and unique uses of drugs. A database, although currently quite limited, tracks the participation of women in clinical trials funded by federal agencies, industries, and nonprofit groups. Federal regulations have recently changed. Additional changes in access to all phases of clinical trials and enhanced monitoring of clinical trials are recommended.