With recent scientific advances leading to better understanding of the immunobiology of recurrent spontaneous abortion (RSA), interest has now focused upon the epidemiology of RSA. A cohort of 214 couples with a history of two or more consecutive abortions were studied for the prevalence of etiologic factors and association with other reproductive failures. The prevalence of causes of RSA in this cohort was compared with etiologic factors among 179 couples with a history of three or more consecutive abortions. The obstetrical histories of 214 women with RSA were analyzed for the total number of pregnancies, live births, stillbirths, spontaneous abortions, ectopic pregnancies, and hydatidiform moles. These numbers were compared with the expected frequency of each in the general population. The prevalence of etiologies among 214 with RSA were as follows: chromosomal-6%, anatomic-1%, hormonal-5%, immunologic-65%, and unexplained-23%. No differences in the prevalence of etiologic factors exist when couples with a history of two or more abortions are compared with three or more abortions. When the number of ectopic pregnancies, molar pregnancies, and stillbirths among 214 women with RSA were compared with the expected numbers, the odds ratios were 2.2 for ectopic pregnancies, 6.0 for molar pregnancies, and 2.3 for stillbirths. These data indicate that no difference in the prevalence of etiologies of RSA exist when couples with two or more abortions are compared with three or more and a comorbidity between RSA and other types of reproductive failure exists.