Increased frequency of thromboembolic events has been recently observed in patients with β-thalassemia major (TM). Platelet function anomalies including impaired aggregation, increased circulating aggregates, and our finding of shortened platelet survival indicate that platelets may be involved in the hypercoagulability in thalassemia. Consequently, we used a technique based on thin layer chromatography purification and enzyme immunoassay to measure urinary metabolites of thromboxane A₂ (TXA₂) and prostacyclin (PGI₂) in nine splenectomized patients with β-TM regularly transfused, five non-splenectomized patients with β-thalassemia intermedia (TI), and 20 healthy individuals. A significant 4- to 10-fold increase was observed in the urinary excretion of 2,3-dinor-TXB₂, 11-dehydro-TXB₂ and 2,3-dinor-6-keto-PGF_1α in patients with TM and TI as compared with healthy controls. No significant differences were found in the concentrations of these metabolites between TM and TI patients. Six TM patients received a very low dose of aspirin (20 mg/day) for 7 days. A significant decrease was observed in the urinay concentrations of 2,3-dinor-TXB₂ and 11-dehydro-TXB₂ derived from platelets. However, the levels of urinary 2,3-dinor-6-keto-PGF_1α reflecting vascular production and TXB₂ and 6-keto-PGF_1α originating from the kidney were not significantly changed. These results are consistent with those of increased in vivo production of TXA₂ because of endogenous platelet activation.