This investigation examined the association between pelvic endometriosis and altered basal body temperature (BBT). The study population consisted of infertile women who have been diagnosed as having endometriosis. A significant association was found between the presence of pelvic endometriosis (without previous treatment) and the appearance of a late decline in BBT during the early follicular phase of the menstrual cycle. A temperature greater than 97.8 degrees F on the first 3 days of menses is associated with pelvic endometriosis. The findings of this study support the early clinical diagnosis of endometriosis in infertile women. Basal body temperature chart analysis may be useful as a clinical adjunct when endometriosis is suspected.