Of 86 medical patients with a hospital discharge diagnosis of thrombophlebitis of the lower extremity, 70 (81%) were of blood type A,B, or AB (non-O). Of the 344 reference patients matched for age, sex, and country, 187 (54%) were of non-O blood type. The estimate of risk ratio of non-O blood type patients to O blood type patients was 3.7 with 90% confidence limits 2.3 and 6.0. The association between non-O blood type and thrombophlebitis of the lower extremity was strongest among the younger patients and women.