Although acute urinary tract infections are common in young women, the associated risk factors have not been defined prospectively.

We recruited sexually active young women who were starting a new method of contraception at a university health center or a health maintenance organization (HMO) and monitored them for six months for symptomatic urinary tract infections. Daily diaries and serial interviews were used to collect data on potential risk factors.

Among 796 women, the incidence of urinary tract infections per person-year was 0.7 in the university cohort (mean age, 23 years; n = 348) and 0.5 in the HMO cohort (mean age, 29; n = 448). In both cohorts, there were strong dose–response relations between the risk of infection and both recent use of a diaphragm with spermicide (respective relative risks for one, three, and five days of use in the past week, 1.42, 2.83, and 5.68 in the university cohort, P<0.001; and 1.29, 2.14, and 3.54 in the HMO cohort, P = 0.04) and recent sexual intercourse (respective relative risks for one, three, and five days with intercourse in the past week, 1.37, 2.56, and 4.81 in the university cohort, P<0.001; and 1.24, 1.91, and 2.96 in the HMO cohort, P = 0.002). The risk of acute infection was also associated with a history of recurrent infection (relative risk, 5.58 in the university cohort and 2.10 in the HMO cohort) but not with cervical-cap use, ABO-blood-group nonsecretor phenotype, or delayed postcoital voiding.

Among sexually active young women the incidence of symptomatic urinary tract infection is high, and the risk is strongly and independently associated with recent sexual intercourse, recent use of a diaphragm with spermicide, and a history of recurrent urinary tract infections.