Twenty-three women with laparoscopically diagnosed early-stage endometriosis collected daily saliva samples at home and brought them to the laboratory at the completion of each menstrual cycle. Levels of progesterone in the saliva were determined by enzymeimmunoassay. Samples were collected for three cycles by one patient, for two cycles by 15 patients and for one cycle be seven women, resulting in a total of 40 cycles for analysis. Only 20 (50%) cycles were classified as normal. In 18 cycles (45%) progesterone levels exceeded the 95th percentile of the normal range. These high levels occurred in the follicular phase only in seven (18%) cycles, in the luteal phase only in eight (20%) and in both follicular and luteal phases in three (7.5%) cycles. Three cycles were anovulatory. One of these cycles also demonstrated elevated progesterone levels in the follicular phase and is therefore included in the 18 cycles mentioned above. We conclude that ovarian function is altered in a significant number of infertile women with endometriosis. However, these alterations are often subtle and only detected by detailed investigation.