A clinical study was made on the relationship between the blood type P1 antigen and urinary tract infection (UTI). The blood type P1 antigen could be detected in 3 out of 11 healthy Japanese volunteers (27.2%), and in 54% of the UTI patients as a whole. Classified by the type of infection, it could be detected in 3 out of 4 patients with upper UTI (75%) and in 11 out of 22 patients with lower UTI (50%). These incidences were higher than that of healthy volunteers, the difference being statistically significant. The relationship between the annual frequency of UTI and the positive detection of P1 antigen was examined. The patients who had been exposed to UTI twice or more a year proved to have a higher detection rate (61%), than the other group of patients, the difference being statistically significant. Two of the patients with E. coli detected as a clinical isolate proved to have the P1 antigen.