To investigate the relation of chlamydial infection of the cervix to time elapsed from the onset of menses, we prospectively studied 338 women receiving routine gynecologic care in two clinical sites in which the prevalence of chlamydial infection was 3 and 6%. Eleven (13%) of 86 women evaluated in the fourth or fifth week after the onset of menses (days 22-35) had chlamydial infection compared to 4 (1.6%) of 252 women seen in the first 3 weeks (days 1-21) (p less than 0.001). For women seen in the fourth or fifth week, the relative risk of chlamydial infection was 8.1 (95% CI, 3.1-20.6). Controlling for known risk factors for chlamydial infection with multiple logistic regression analysis, the relative risk was sustained (RR 10.8; 95% CI, 3.1-37.4). We conclude that time elapsed from the onset of menses is an important independent risk factor for chlamydial infection of the cervix that may lead to more effective and efficient screening strategies.