Forty‐eight acne patients were treated orally with 13‐cis‐retinoic acid in a double‐blind dose response study. There was a marked clinical improvement with a concomitant reduction in sebum excretion rate (SER) and production rate of free fatty acids (FFA). Microbial numbers decreased significantly; the decrease in propionibacteria was greater than that of aerobic bacteria. The decline in micro‐organisms occurred after the reduction in sebum and FFA production. This suggests that the effect of the drug upon micro‐organisms is secondary to the change in sebum excretion but it is nevertheless an important factor in the resolution of the acne.