The composition of human sebum varies among individuals with respect to the proportions of various types of fatty acids in the ester lipid classes. Iso-branched fatty acids with even numbers of carbons (iso-even) are particularly variable. However, in 13 identical twin pairs, intrapair differences in the proportions of iso-even fatty acids in sebaceous wax esters were found to be very small, while interpair differences were as large as in the nontwin population. Intrapair differences in iso-even fatty acids were also quite small in 4 of 8 fraternal twin pairs. Heritability estimates suggested that proportions of iso-even fatty acids in human sebum are controlled, at least in part, by genotype.