We aimed to investigate the prevalence of ulcerative sexually transmissible diseases (STDs) and hepatitis in crack users. We interviewed 435 crack users on site in crack houses in Houston, Texas and took blood for laboratory analysis. There was evidence of syphilis infection in 13%, herpes simplex virus-2 (HSV-2) in 61%, HIV in 12%, hepatitis B in 52%, and hepatitis C in 41% of cases. On DSM-III-R criteria, 12% were crack abusers and 84% crack dependent: over half reported previous treatment. Forty per cent reported also injecting. Sexual behaviour indicated a mean of 2.4 partners in the past month for men, 3.7 for women. Sexual behaviour was largely vaginal, although women also reported more than twice the level of oral sex of men. Significant multivariate predictors for HIV and hepatitis B and C were previous reported STD and injecting drug use (including sharing needles), while female gender for syphilis and HSV-2, and additionally condom use for HSV-2, were significant risks. These data confirm high rates of STDs in a crack house population as inferred from previous clinic-based and community studies, and the link between STDs, injecting and HIV. The high rates of STDs found should lead to considering STDs and substance abuse to be dual diagnoses in crack users and the integration of STD diagnosis and treatment into crack outreach and treatment programmes.