To test the hypothesis that the seborrhoea of acne is an end-organ hyper-response to androgens, a standardized dose of 2% testosterone cream was applied to the forehead of eight prepubertal boys for 6 weeks and the effect determined by measurements of sebum excretion rate (SER). Three boys showed a 15-fold increase in SER and three boys were non-responders, whereas two boys had SER values between those of the responders and non-responders. These data show that sebaceous glands are stimulated by androgens to varying degrees and support the theory of an end-organ response in acne.