The genetically determined acetylator phenotype in diabetic children with and without increased urinary albumin excretion was investigated. Acetylator phenotype was determined according to Evans, and 24-hour albumin excretion rate (AER) was measured by immunoturbidometry in 86 children and adolescents with type 1 (insulin-dependent) diabetes mellitus and in 100 age-matched healthy controls. In diabetics, the fast acetylator phenotype was found in 36 (41.9%) patients and the slow one in 50 (58.1%); the control group had 52 (52%) fast and 48 (48%) slow acetylators. There were no significant differences in acetylator phenotypes between diabetic patients and control subjects (chi 2 = 1.0, NS). Among patients with normal albumin excretion (n = 70, mean age: 12.9 +/- 3.5 years, mean diabetes duration: 5.3 +/- 3.8 years, AER < 20 micrograms/min), 35 (50%) fast acetylators and 35 (50%) slow acetylators were found. In patients with elevated albumin excretion (n = 16, mean age: 14.0 +/- 3.2 years, mean diabetes duration: 4.9 +/- 3.0 years, AER > 20 micrograms/min), 1 (6.3%) patient was a fast acetylator and 15 (93.7%) were slow acetylators. A significant difference has been found between the two groups in the rate of fast/slow acetylators (chi 2 = 8.79, p < 0.01). The strong correlation between the slow acetylator phenotype and microalbuminuria in diabetics suggests that: (a) genetic factors may play a role in the development of diabetic nephropathy; (b) the acetylator status could be a useful tool to detect patients 'at risk' of nephropathy.