In patients with diabetes mellitus, contradictory results have been reported indicating both increased and reduced risks of malignancies. In the present trial all insulin-treated diabetic patients (n = 2720) attending our centre since 1995 were studied. Of these patients, 28 (type 1/type 2: n = 1/27, 23 women) developed malignancies during insulin therapy: 11 patients developed cancer of the breast, 4 patients cancer of the pancreas, 3 patients cancer of the kidneys and 10 patients developed other malignancies. The characteristics of these patients [mean +/- SD (range)] were as follows: age 68.8 +/- 8.6 (52.0 87.0) years, diabetes duration 13.1 +/- 8.1 (0.5-29.0) years, duration of insulin therapy at the time of the diagnosis of malignancy 4.3 +/- 5.7 (0.5 24.0) years, insulin dosage 0.67 +/- 0.43 (0.11-1.72) IU/kg body weight, mean HbAlc 9.6 +/- 1.9 (6.8-14.9)% (HPLC, Diamat, normal range 4.4%-5.9%). The prevalences of nephropathy, retinopathy (non-proliferative: n = 7) and peripheral neuropathy were 35.7%, 25.0% and 46.4% respectively. When the features of the 27 patients with type 2 diabetes were compared with the characteristics of the type 2 diabetic patients (n = 117, 63 women) studied in a population-based survey of insulin-treated diabetic patients, also performed in the area of Jena [JEVIN; Schiel R et al. (1997a)] there were no significant differences in the duration of insulin therapy (JEVIN: 4.7 +/- 4.3 years, P = 0.64), insulin dosage (JEVIN: 0.55 +/- 0.27 IU/kg body weight, P = 0.08), mean HbAlc (JEVIN: 9.0 +/- 2.1%, P = 0.16) and the prevalences of long-term complications of diabetes. The quality of diabetes control in insulin-treated patients suffering from malignancies is comparable to that of a selection-free population of diabetic patients. Furthermore, in comparison to non-diabetic subjects our diabetic patients showed no altered risk for malignancies as a function of insulin dosage, the duration of diabetes or insulin therapy, the quality of diabetes control or the prevalence of long-term complications of the disease.