Gonorrhea is an important marker for endocervical chlamydial infections in nonpregnant women. Concomitant infection rates as high as 50% have been reported. There are few data on concomitant infection rates in pregnant patients. The purpose of this study was to examine the prevalence of endocervical chlamydial infections in pregnant women with gonorrhea. Patients with cervical cultures positive for Neisseria gonorrhoeae at their initial prenatal visit had endocervical specimens for Chlamydia trachomatis culture obtained before anti-gonorrheal therapy. Control patients were selected at random from the same prenatal population. The prevalence of C trachomatis in patients with gonorrhea was significantly greater than that in the control population (46 versus 5%; P less than .001). Patients with gonorrhea were younger, less often married, and more often black than the control population, but these demographic differences did not account for the large difference in the chlamydial prevalence. Erythromycin 500 mg four times daily provided an excellent cure rate without intolerable side effects. Pregnant patients being evaluated or treated for gonorrhea should also be considered at high risk for concomitant cervical chlamydial infection.