Summary.Acetylator status was determined in 112 normal and 116 diabetic subjects. Forty-eight percent of the normal subjects were fast acetylators compared with 74% of the Type 1 (insulin-dependent) diabetic and 54% of the Type 2 (non-insulin-dependent) diabetic subjects. This result for Type 1 diabetic patients was significantly different from the normal subjects, while that for Type 2 patients was intermediate between the two. In contrast to a previous report, there was no significant association of acetylator status with peripheral neuropathy in subjects with either Type 1 or Type 2 diabetes. Acetylator status could be a genetic marker for Type 1 diabetes, but the increased proportion of fast acetylators in both groups suggested the possibility of an artefact due to high glucose levels. The rate of acetylation of sulphadimidine was significantly greater in both fast and slow acetylators with diabetes compared with normal subjects and there was a significant association between high plasma glucose and acetylator status. Prospective studies will be necessary to confirm whether acetylator status is a true genetic marker for diabetes.