OBJECTIVE To estimate the prevalence of sexually transmitted diseases (STDs) and the acceptability of STD screening among people seeking an HIV antibody test in an established free standing HIV testing clinic. METHODS A 9 month period prevalence study conducted between August 1993 and April 1994. METHODS The Same Day Testing Clinic (SDTC) for HIV antibodies at the Royal Free Hampstead NHS Trust Hospital, London. METHODS 242 males and 160 females attending the Same Day Testing Clinic. METHODS The prevalence of STDs including gonorrhoea, chlamydia, syphilis and hepatitis B and the percentage of clinic attenders accepting an STD screen. RESULTS Of those invited to take part in the study 69% of the males (242/350) and 59% (160/269) of the females agreed to be screened although for a variety of reasons not everyone agreed to a full screen. Two cases of untreated syphilis, no cases of gonorrhoea and six cases of chlamydia were detected. Four people had active, previously undiagnosed herpes while three had genital warts. Evidence of previously unknown hepatitis B infection was found in 26 people. Despite a high level of previous contact with genitourinary medicine services, uptake of hepatitis B vaccination among those homosexual men eligible for immunisation was low (28%; 23/83). Nine (4%) of the males, but none of the females screened for STD were found to be HIV antibody positive. CONCLUSIONS Among people seeking an HIV antibody test in an established free standing HIV testing clinic, the prevalence of acute STDs was low. However, evidence of previously undiagnosed hepatitis B infection was found in a number of subjects and uptake of vaccination among those most at risk had been low. While opportunistic screening for STD was acceptable to almost two thirds of HIV testing clinic attenders, a substantial minority nonetheless declined this offer. Selective STD screening could be offered to those people seeking an HIV test who report never having been screened before, as both cases of positive syphilis serology and all those of chlamydia were in people who had not previously been screened. All those at risk for hepatitis B infection should be encouraged to establish their infection status and be immunised where appropriate.