A genetic variation in the prothrombin gene is located in the 3-untranslated region at position 20210 where a G-->A transition occurs. The prevalence of the mutation is 1% to 2% in white populations, and the mutation is associated with an increased risk of venous thrombosis and myocardial infarction. We report the prevalence of the A allele in blacks at birth; in black control subjects with no history of heart attack, stroke, or blood clots; in black patients with venous thrombosis; and in black patients with myocardial infarction. Among 318 infants, the prevalence of the A allele was 0.2% (1 heterozygote), with an exact, one-sided upper 95% confidence limit of 0.7%. Among 185 control subjects the variant was absent, yielding an exact, one-sided upper 95% confidence limit of 0.8% for the A allele. The heterozygous genotype was found in 2 of 91 subjects with deep vein thrombosis and in none of 123 subjects with myocardial infarction. The very low prevalence of the A allele indicates that the prothrombin variant is not a major cause of venous thrombosis or myocardial infarction in blacks.