This study describes the effects of nicotinamide therapy on B-cell function in Type 1 (insulin-dependent) diabetes. C-peptide secretion was studied in 20 patients newly diagnosed with Type 1 diabetes at basal state and also after an i.v. glucagon stimulus. Patients were randomly allocated according to a single-blind schedule, to one of the following treatments over a 45-day period: Group 1: 10 patients, nicotinamide 1 g/day; Group 2: 10 patients, placebo. The C-peptide secretion tests were performed before treatment and on days 15, 45, 180, 365 of the follow-up. The clinical and metabolic data were similar in the two groups of patients. Basal and stimulated C-peptide levels increased by 45 days in both groups, but the increase in stimulated C-peptide response was greater in the nicotinamide group (p less than 0.01). However, the B-cell function decreased after the period of nicotinamide administration. No difference in the number of clinical remissions or insulin requirement and HbA1 between the groups was observed. These data suggest that treatment of Type 1 diabetes with nicotinamide at diagnosis is associated with a moderate increase of C-peptide secretion recovery.