Two hundred sixty-three patients with a diagnosis of endometriosis or adenomyosis were studied in a retrospective fashion. The association between endometriosis resulted in a spontaneous abortion rate of 44.3%, Patients with adenomyosis spontaneously aborted 35.1% of their pregnancies. These figures are significantly increased in comparison to those pertaining to the general population. Uterine pathology occurring with endometriosis is discussed in relation to secondary infertility. Hypotheses that may explain the relationship between endometriosis and spontaneous abortion are discussed briefly.