The factor V Leiden mutation and a guanine-to-adenine mutation at nucleotide 20210 in the 3'-untranslated region of the prothrombin gene are the most prevalent genetic defects in patients with deep venous thrombosis. Heterozygosity for the factor V Leiden and the prothrombin gene mutations is found in approximately 20 and 6% of unselected patients with deep venous thrombosis, respectively, whereas the prevalences of the two mutations in the general Caucasian population are approximately 6 and 2%, respectively. We evaluated an 18-year-old man presenting with a spontaneous episode of superficial venous thrombosis for the presence of an inherited thrombotic disorder. After excluding deficiencies of antithrombin, protein C, and protein S, genomic DNA from the patient was tested for the presence of the factor V Leiden and prothrombin gene mutations. Consanguinity was not present in the family. Genotyping demonstrated that the patient was homozygous for the factor V Leiden and prothrombin gene mutations. The likelihood of identifying an individual in the general population who is homozygous for both mutations similar to our patient is estimated to be less than 1 in 10 million.