Hepatic acetylator phenotype was determined using the sulphamethazine method in 32 patients with juvenile-onset diabetes mellitus and in 87 patients with the maturity-onset diabetes mellitus (Types I and II) and among 157 controls. No statistical differences were observed, neither for the incidence of acetylator phenotype among diabetics as compared with the control group, nor for the age at diagnosis of maturity-onset diabetes in slow and rapid acetylators. The incidence of diabetic retinopathy was similar among both phenotypes.