Adherence and retention of female injection drug users (IDUs) in clinical trials are not well known and were evaluated among 458 female IDUs in a clinical trial in Baltimore. Of all, 62.9% were adherent to visits (attended ≥ 80% of visits). Of women with ≥ 1 visit after enrollment, 76% were adherent to treatment (took ≥ 80% of pills); 27.7% were lost to follow-up (missed ≥ 3 consecutive visits). Women nonadherent to visits were younger and less likely to be on methadone. Women lost to follow-up were younger, more often white, not on methadone, and injecting drugs daily. Fair-moderate adherence to visits and treatment occurs among female IDUs in a clinical trial.