Summary.— The sebum excretion rate and skin surface lipid composition were determined in 193 male and female control subjects aged between 4 and 60 years. The sebum excretion rate, squalene and wax ester content of surface lipid were maximal in males aed 26–40. The percentages of surface lipid wax esters were greater in males than females at all ages, but the prepubertal peak of wax esters in males may be due to an epidermal contribution from sterol esters. There was no significant age or sex variation in the sum of the percentages of triglycerides and free fatty acids, but the degree of hydrolysis varied considerably with age. The free fatty acid content was greatest in the first decade in both sexes, when the sebum excretion rate is at the lowest levels. Cholesterol content of surface lipid rose to maximal levels at puberty in both sexes.