Forty patients with cystic fibrosis (CF), including 34 who died above age 10 years without having developed clinical diabetes mellitus and 6 who died with both cystic fibrosis and diabetes mellitus, were studied. The mean age of the female patients with CF but not diabetes was 15.8 +/- 5.6 years; of males without diabetes, 17.2 +/- 6.4 years; of female patients with CF and diabetes mellitus, 20.2 +/- 6.9 years; and of males with CF and diabetes, 21.3 +/- 6.6 years. The mean number of pancreatic islets in microscopic sections for patients with cystic fibrosis but not diabetes was 4.18 +/- 2.76/mm2, and the value for patients with both cystic fibrosis and diabetes mellitus was 2.61 +/- 2.07/mm2. The lowest density of pancreatic islets (1.69 +/- 0.48/mm2) for cystic fibrosis was found in patients with the latest-stage pathologic lesion. Nesidioblastosis (presence of ductuloinsular complexes) was identified in 14 of 38 cystic fibrosis patients, both with and without diabetes mellitus. The pancreatic islets of both diabetic and nondiabetic patients with CF showed hypertrophy; the mean volume of the three largest pancreatic islets for CF only was 0.0117 +/- 0.00657 mm3 and that for cystic fibrosis and diabetes was 0.00795 +/- 0.00599 mm3, both values being larger than normal. Ratios of the amounts of islet endocrine cells, A cells, B cells, and D cells, were determined by peroxidase--anti-peroxidase labeled antibody staining. The B cells composed 43.0% of endocrine cell mass in cystic fibrosis alone and 30.1% in cystic fibrosis with diabetes mellitus, which were lower than normal proportions. The D cell values, 11.9% in cystic fibrosis and 15.1% in cystic fibrosis with diabetes mellitus, on the other hand, were greater than normal ratios.