Basal body temperature of 168 cycles have been measured in 20 infertile women (age 21-31 years) with endometriosis. There were 10.1% anovulatory cycles, and in 22.5% of the 151 biphasic cycles the hyperthermic phase lasted less than 11 days. A late decline of basal body temperature after the onset of menstruation was recorded in 34.5% and may be only limitedly indicative of endometriosis. In 64.9% of all cycles basal body temperature was somewhat elevated and/or proved one or more temperature peaks in the follicular phase. Perhaps this may be indicative of endometriosis. Nevertheless the so-called endocrinologic infertility requires endoscopy for accurate diagnosis.