Hypervolemia is considered to play a major role in the pathogenesis of diabetic vasculo- and nephropathy. The aim of our study is to determine whether children and adolescents with insulin-dependent diabetes mellitus (IDDM) experience alterations in blood volume (BV) before onset of apparent nephropathy. BV (calculated as the sum of measured plasma volume (PV) and red cell volume (RCV)) was determined in 31 children (9-16 yr) with a mean duration of IDDM of 6.6 yr and without microalbuminuria. Due to dependence of these values on age, size and sex, all data were normalised for body size parameters. While no statistical difference for BV normalised for lean body mass (LBM) (86.98±9.5 ml/kg) was found in diabetic children compared with our control population (84.91±12.08 ml/kg), a difference could be shown when normalised for body surface area (BSA) (diabetic children 2.37±0.3 L/m2; control population 2.15±0.38 L/m2, p=0.002). Increased BV is only present when normalising for BSA and not for the theoretical superior LBM-index. Because the study population exhibited a poor glycemic control (HbA1c 10.2±2.4 %), an influence of glucosuria-induced polyuria on BV cannot be excluded. Taking into account these limitations our data do not confirm the presence of hypervolemia before onset of diabetic nephropathy.