Human serum contains several glycosaminoglycans (GAGs), mainly chondroitin sulphates and significantly less of heparan sulphate + heparin and dermatan sulphate. The non-insulin-dependent diabetes mellitus (with vascular complications) was associated with a significant increase in total serum GAG concentration, mainly of chondroitin sulphates and dermatan sulphate, with a simultaneous decrease in heparan sulphate + heparin level. These alterations were much more evident in patients with poor metabolic control. Hyaluronic acid (undetectable in healthy subjects and in patients with good metabolic control) appeared only in trace amounts in poorly controlled diabetic individuals. The obtained data allow to conclude that the diabetes mellitus-associated disturbances in tissue GAG metabolism lead to significant alterations in serum GAG composition.