To determine factors influencing the enrollment of women in a large multicenter human immunodeficiency virus (HIV) clinical trials program in the United States, we analyzed enrollment and demographic data of the AIDS Clinical Trials Group (ACTG) during the period 1987-90. Women comprised 6.7% of 11,909 ACTG participants enrolled in 1987-90. Women entering ACTG trials were significantly more likely to be white (48.5%) and less likely to have ever used i.v. drugs (22.6%) than U.S. women reported to have AIDS (26.5% were white; 51.0% had ever used i.v. drugs, p < 0.0001). In a multiple logistic regression model, specific attributes of individual trials did not influence enrollment of women with the exception that trials that targeted asymptomatic persons had greater enrollment of women. There was wide variation among research units in the percentage of women enrolled (1.0-37.5%), and evidence of significant regional variation in the ability of units to recruit available women. Units with female principal or coprincipal investigators had more than twice the percentage of female enrollment as units headed by men (10.8 vs. 5.3%, p < 0.001). Enrollment of women in a large HIV clinical trials program was low and appeared to be influenced more by demographic and geographic factors that attributes of specific trials. An apparent positive influence of female leadership on the enrollment of women warrants further study.