The influence of skinfold thickness on the local subcutaneous blood flow (SBF) and absorption rate of subcutaneously injected and infused soluble insulin was investigated in insulin-dependent diabetic patients. The local 133Xenon washout technique was used for measuring SBF and the disappearance of 125I labelled insulin for measuring insulin absorption. A Harpenden skinfold caliper was used for measuring skinfold thickness. A large variation in skinfold thickness (range 6-40 mm, N = 50) was demonstrated. The SBF was curvilinear related to the skinfold thickness with decreasing SBF for increasing skinfold thickness (N = 50). A similar relationship was found between the SBF and the absorption rate of subcutaneously injected (N = 10) and infused (N = 7) insulin. The large variation in skinfold thickness and thus insulin absorption may partly explain the well-known large inter-individual variation in insulin absorption, and the different absorption rates from areas with different skinfold thickness should be taken into account when treating with insulin.