Patients who are homozygous for the G to A nontranslated prothrombin polymorphism only occasionally have venous thrombosis. An evaluation of all published papers on the subjects has disclosed that nine patients of the 35 so far reported remained asymptomatic in spite of the presence of associated congenital or acquired thrombotic risk factors. We saw an additional patient recently, bringing the total to 10 of 36 patients. Some of these patients remained asymptomatic in spite of multiple or repetitive risk factors (e.g., five pregnancies in the case of one patient). Twelve patients who were homozygous and who had this polymorphism developed symptoms only in the presence of the same risk factors. This may suggest that this abnormality played a small role, if any, in both groups of patients. The finding that several patients with this abnormality remained asymptomatic in spite of associated risk factors casts serious doubts about the prothrombotic significance of this polymorphism. Until this problem is clarified, the clinician must abstain from attributing a prothrombotic effect to this polymorphism.