The distribution of N-acetylation phenotypes has been investigated in 410 consecutive patients prior to diagnostic excision of a breast lump and compared to that in 337 healthy controls. Contrary to previously published data there was no excess incidence of fast acetylators in patients with malignant breast disease. There was, however, a significant trend for more rapid acetylator ratios to be associated with advanced disease at first presentation. There was a slight but non-significant excess of fast acetylators in patients with benign breast disease as compared to controls, but there were no significant differences in the proportion of the acetylator phenotypes between cystic breast diseases with or without epithelial hyperplasia. There were no significant associations between oestrogen and progestogen receptors and the acetylator phenotype. It is concluded that the acetylation phenotype has no important association with malignant breast disease.