A family study of hidradenitis suppurativa was undertaken based on 26 subjects with the disease. The probands were obtained from Hospital Activity Analysis (HAA) records for a three year period (1980 to 1983) and by direct referral from hospital specialists over a six month period (1983 to 1984). Family pedigree information was collected by home visits or hospital interviews and confirmation of the disease in relatives was obtained by examination where possible or by telephone contact and hospital/GP records. A total of 62 affected persons was eventually ascertained, 40 females and 22 males. In 11 families there was evidence in favour of a genetic aetiology with single gene transmission. In another three families there was historical evidence of familial occurrence and in nine families no family history was obtained at the time of enquiry. Problems of ascertainment, including variability of age of onset and psychosocial factors, were noted and could be responsible for false negative family histories or underestimation of affected persons.