In homocystinuria, thrombosis is frequent (50% of the cases). The authors report a case of cystathionine-synthetase deficiency-induced homocystinuria associated with thrombosis of a common iliac vein. This patient presented with a blood coagulation disorder which was related to severe factor VII deficiency and which was corrected by treatment. Homocystinuria was pyridoxine independent as witnessed by the fact that Brand's reaction remained positive with vitamin B6 therapy and cystathionine. Cystathionine-synthetase activity measured in cultured fibroblasts increased only slightly with pyridoxine therapy. Treatment (Betaine, folic acid, vitamin B 12) induced the disappearance of plasma methionine. Clinically, the patient whose treatment came late in the course of her disease, was only slightly improved. Hypotheses about etiopathogenic mechanisms of this disease are discussed. Difficulties of treatment and diet are reviewed. The relative inefficacy of therapy, even in case with early treatment, makes antenatal diagnosis preferable to neonatal diagnosis.