Pancreatic tissue obtained by hemipancreatectomy from healthy living related donors has been transplanted into recipients with Type I diabetes mellitus. To determine the metabolic consequences of this procedure for the donors, we carried out oral glucose-tolerance testing and 24-hour monitoring of serum glucose levels and urinary C-peptide excretion as a measure of insulin secretion in 28 donors, both before and one year after hemipancreatectomy. The mean fasting serum glucose level was significantly higher one year after the procedure (mean ±SD, 5.4±0.9 vs. 4.9±0.5 mmol per liter; P<0.003), as was the serum glucose value two hours after the administration of glucose (8.7±2.9 vs. 6.5±1.0 mmol per liter; P<0.001). The fasting serum insulin level was significantly lower one year after hemipancreatectomy (33.0±21.6 vs. 38.4±21.6 pmol per liter; P<0.05), as was the area under the insulin curves during the oral glucose-tolerance test (52,554±22,320 vs. 76,230±33,354 pmol per liter per minute; P<0.04). The mean 24-hour serum glucose—profile value was higher at one year, and the 24-hour urinary C-peptide excretion was lower in the 17 donors who underwent these studies. Seven of the 28 donors had abnormal glucose tolerance one year after hemipancreatectomy; however, insulin secretion in these 7 donors was indistinguishable from that in the 21 donors who had normal glucose tolerance. All 28 donors had fasting serum glucose concentrations lower than 7.8 mmol per liter, and their mean 24-hour plasma glucose levels remained within the normal range. We conclude that in healthy donors hemipancreatectomy results in a deterioration of insulin secretion and glucose tolerance, as measured one year later. Further study is required to ascertain whether the development of clinical diabetes mellitus is a risk inherent in hemipancreatectomy.