The pattern of spermaturia in boys at different stages of puberty was investigated. Fractionated 24 hour urine was collected for nine consecutive days from eight boys aged 13-14 years and 10 boys aged 15-17 years. Spermatozoa were detected by microscopic examination of the sediment. Sex characteristics were recorded. Fifty five per cent of all urine samples were positive for sperm and all boys showed spermaturia. A large variation in spermaturia was found between and within boys at the same stage of puberty. Spermaturia was a more common and regular event during early and mid-puberty than in more mature subjects. This indicates that the mechanism of spermaturia in early and late puberty could be different. It is suggested that spermaturia in non-virilised boys could be a result of a spontaneous, continuous flow of spermatozoa to the urethra in contrast with the peristaltic flow during ejaculation occurring at a later stage of puberty.