Acne is principally a disorder of adolescence but persists into middle age in a small minority of individuals. There is some evidence, primarily from twin studies, to suggest that acne may be an inherited disease. We have carried out an investigation of the familial risk of persistent adult acne by studying the occurrence of this condition in first-degree relatives of patients with adult persistent acne compared with the relatives of unaffected matched controls. Two hundred and four patients over the age of 25, referred to our department with facial acne which had persisted from adolescence, were included in the study. For each patient, a detailed pedigree which included all first-degree relatives was drawn up. For all relatives over the age of 25, demographic details and the presence or absence of facial acne was recorded. One hundred and forty-four unaffected controls were recruited. These were matched with the patient group in terms of age, social class and ethnic origin. The same questions were asked of controls as of the cases, and they provided the same information on their first-degree relatives. In total, 204 acne cases and 144 non-acne control volunteers were studied, contributing 1203 and 856 first-degree relatives, respectively. Two hundred and three first-degree relatives of cases were affected with facial acne, compared with 42 first-degree relatives of volunteers. The risk of adult acne occurring in a relative of a patient with adult acne was significantly greater than for the relative of an unaffected individual (odds ratio 3.93, 95% confidence interval 2.79-5.51; P < 0.001). Our study suggests that familial factors are important in determining individual susceptibility to adult persistent facial acne. Genetic factors may determine the failure of acne-prone follicles to evolve into acne-resistant follicles in early adult life.