Nicotinamide, a poly-(ADP-ribose) synthetase inhibitor, has been shown in animal models to induce islet B-cell regeneration. An open controlled trial was therefore carried out for 1 year in 36 patients with recent onset (less than 4 weeks symptoms) Type 1 diabetes. Twenty-three patients were treated with nicotinamide (200 mg daily) in addition to insulin, and 13 control patients were treated with insulin alone. Metabolic and immunological variables at entry were similar in the two groups. A significant increase of stimulated plasma C-peptide levels compared to diagnosis was observed only in the nicotinamide treated group (1.4 +/- 0.3 (+/- SE) micrograms l-1 at diagnosis vs 2.4 +/- 0.4 at 6 months, p less than 0.04; and 3.0 +/- 0.5 at 1 year, p less than 0.01). Patients receiving nicotinamide had lower glycosylated haemoglobin levels at 6 months and 1 year compared to the control group (p less than 0.04 and p less than 0.03, respectively) although insulin dose was lower. Small doses of nicotinamide may be successful in improving metabolic control in recent onset Type 1 diabetes, probably by increasing residual islet B-cell function.