An anonymized sero-survey of the prevalence of HIV antibody was performed at an inner city Genitourinary medicine clinic in Birmingham. In 1991 8686 patients undergoing routine serological syphilis tests were anonymously tested for HIV antibodies once during the year. Demographic information was recorded for each sample but they were otherwise unlinked. There were 31 samples which tested positive for anti-HIV 1 from this group compared with 13 diagnosed by concomitant voluntary named testing. Sero-prevalence rates of 0.17% for women and heterosexual men and 4.37% for homosexual/bisexual men were found. No drug users tested positive. The survey provided evidence of occult disease outside the recognized risk behaviour patterns of homosexual men and injecting drug users outside London.