The eleventh known kindred with a vertical pattern of familial male sex precocity is described. Linkage data on blood groups and the phenylthiourea (PTC) taste test were obtained. There is limited evidence for linkage between the Kidd (Jkb) blood groups and the syndrome of familial male precocity. No urinary FSH could be detected in any of the affected children so tested. There was no evidence of ICSH activity in the urine of the propositus, which is against the hypothesis that this syndrome is due to overproduction of ICSH. The urinary C19 steroids of the propositus and his father (also affected) were fractionated on a chromatographic column. The propositus showed dehydroepiandrosterone (DHA) in his urine, whereas normal boys of a similar age did not. The urinary androsterone (ANDRO) in the affected boy was 4 times the concentration in the urine of the control boys. His urinary etiocholanolone (ETIO), 11-OH-ETIO, 11-O-ETIO and 11-OH-ANDRO were about double the excretion of these steroids by the normal boys. Paradoxically, the father excreted less of these C19 steroids, except for 11-O-ETIO, than did the control men.