Reversible aggregation of red blood cells (RBC) plays an important role in determining blood flow properties, and it is this aggregation which increases blood viscosity at low shear rates. The structure and sites of venous thrombi, as well as the fact that stasis is a major predisposing factor in venous thrombosis, suggest a strong association between vein thrombosis, slow blood flow and increased blood viscosity. RBC aggregation and disaggregation were measured (SEFAM erythroaggregameter, France) in 54 patients with a history of unexplained leg vein thrombosis. Results were compared to those of controls classified according to age. Increased RBC aggregability was observed in 41% of the patients, and the mean values indicated a significant elevation of RBC aggregability in patients when compared with controls (P < 0.05). Subgroups were compared to study the influence of thrombus recurrence and thrombosis type (deep versus superficial vein thrombosis) on the aggregation parameters. No significant difference was found between these subgroups. The use of compression stockings and veinotropic drugs tended to reduce the abnormalities in RBC aggregability (P < 0.05). An increase in RBC aggregability and in the shear resistance of RBC aggregates, by predisposing to circulatory stasis, is likely to contribute to the evolution and complications of leg vein thrombosis.