In 26 type 2 diabetic patients with failure to diet and sulphonylureas (fasting blood glucose levels greater than 8.0 mmol/l) the effects of insulin therapy on blood glucose control, islet B-cell function and plasma lipids were studied. Age was 58 +/- 11 (SD) yr, median duration of diabetes 6.5, range 1-24 yr, and body mass index 24.5, range 18.9-36.3 kg/m2. Six patients were obese. Therapy comprised twice daily injections of intermediate-acting insulin with additional fast-acting insulin when necessary. After six months, insulin dose was 39 +/- 10 U in the non-obese patients. Their fasting (14.0 +/- 2.7 mmol/l) and post-prandial blood glucose (17.9 +/- 4.5 mmol/l) and glycosylated haemoglobin (HbA1, 13.0 +/- 2.0%) declined to 7.7 +/- 1.6 mmol/l, 10.6 +/- 2.6 mmol/l and 9.5 +/- 1.0%, respectively (p less than 0.001). Median body weight increased by 3.7 kg (p less than 0.001). The changes in body weight correlated well with the changes in fasting blood glucose (r = -0.75, p less than 0.01) and HbA1 (r = -0.73, p less than 0.01). Fasting plasma insulin increased (p less than 0.01), whereas fasting plasma C-peptide and C-peptide release after glucagon did not change. Free fatty acids, LDL-cholesterol, total and VLDL-triglycerides showed a significant (p less than 0.05) decrease during insulin treatment. In the six obese patients insulin dose after six months was 44 +/- 18 U. Fasting blood glucose fell from 11.3 +/- 2.2 to 8.8 +/- 2.7 mmol/l (p less than 0.01), and HbA1 decreased from 10.7 +/- 1.1% to 9.8 +/- 1.3% (p less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)