Although regarded as a benign condition, simple hypertrichosis may be very disturbing to a child cosmetically. An abnormality in androgen metabolism has been implied in the etiology of simple hypertrichosis recently. This study was undertaken to investigate the effect of spironolactone therapy for its antiandrogenic property in 12 prepubertal girls with hypertrichosis with no underlying etiology. The girls, with a mean age of 6.9 (1.2) years, had normal height and bone age. Basal hormone levels and adrenocorticotropin stimulation test results were in the normal ranges. Pelvic and adrenal ultrasound did not reveal pathology. Total and medullary hair width were measured from hair taken from preauricular, anterior midthigh, distal and proximal forearm areas. Spironolactone was started at an oral dose of 25 and increased to 100 mg x m(-2) x day(-1) twice daily for 1 year. Total hair width decreased significantly in the preauricular and anterior midthigh regions up to 6-9 months of treatment. Medullary hair width, which was affected by therapy to a greater extent, decreased significantly in all regions up to 6 months. Both total and medullary hair width showed a tendency to increase afterwards. No side effects were encountered. Spironolactone may be used in the treatment of simple hypertrichosis; however, more data are needed to clarify the efficacy and safety of anti-androgen therapy in hypertrichosis.