AIM: To investigate the influence of obesity in type 2 diabetic patients upon pharmacological properties of different biphasic preparations of insulin aspart. METHODS: A total of 75 type 2 diabetic patients were stratified according to their body mass index (BMI) into 40 non-obese (BMI 23-28 kg/m(2)) and 35 obese (BMI 30-35 kg/m(2)) subjects. The trial was a double-blinded crossover study. In two periods of 4 weeks each the patients received subcutaneous injections of biphasic insulin aspart 50 (BIAsp 50) or biphasic insulin aspart 70 (BIAsp 70) thrice daily in random order. Insulin doses were titrated individually. At the end of each period 24-h serum profiles of insulin aspart, C-peptide and glucose were recorded. The primary endpoint was the area under the curve of serum glucose concentration during 24 h (AUC(Glu)(0-24 h)). RESULTS: The insulin concentration profiles of BIAsp 50 and 70 were as expected according to the content of protamine-bound insulin aspart (50 and 30% respectively). AUC(Glu(0-24 h)) BIAsp 50/BIAsp 70 ratios were 0.97 (95% CI: 0.90-1.05, p = 0.49) for non-obese and 0.98 (95% CI: 0.92-1.05, p = 0.55) for obese. Fasting serum glucose (FSG) BIAsp 50/BIAsp 70 ratios were 0.90 (95% CI: 0.84-0.96, p = 0.002) for non-obese and 0.90 (95% CI: 0.84-0.97, p = 0.006) for obese. During both treatment regimens the frequency of minor hypoglycaemic episodes was highest for the non-obese group. CONCLUSIONS: The pharmacokinetic and pharmacodynamic characteristics of the two preparations of biphasic insulin aspart were different; however, they were not influenced by the degree of obesity in type 2 diabetic patients.