To determine whether concurrent gonorrhea reactivates latent chlamydial infection, we studied 74 recurrent chlamydial infections and the effect of concurrent gonorrhea at the recurrent episode on the chlamydial serovar identified. Serotyping of 74 recurrent pairs of chlamydial isolates from patients attending a sexually transmitted diseases clinic indicated that 47.1% (16 of 34) with gonorrhea at the time of recurrence harbored chlamydiae of the same serovar as at the initial infection, while only 22.5% (9 of 40) without gonorrhea had the same serovar (P = .03). The proportion of recurrences by the same serovar in the group without gonorrhea did not differ from the proportion predicted by a random exposure model (22.2% vs. 18.4%, P = .46), while the proportion in the gonorrhea group did (47.1% vs. 19.8%, P less than .0001). The possibility of reinfection did not appear more likely in the group with gonorrhea than in the group without. These observations support the hypothesis that concurrent gonorrhea can reactivate latent chlamydial infection.