Both hyperprolactinemia and endometriosis are associated with infertility. A study was performed to ascertain whether sleep-related prolactin (PRL) hypersecretion was present in endometriosis. Fifty-five consecutive infertile women with regular menstrual cycles and admitted for diagnostic laparoscopy were studied. Blood samples were drawn throughout the night preceding surgery. Serum PRL, estradiol and progesterone levels were measured with radioimmunoassays. Nocturnal patterns of PRL secretion may be altered in infertile women with endometriosis, with an exaggerated and prolonged nocturnal peak. This alteration in PRL dynamics may contribute to infertility in women with endometriosis and may be a part of the pathophysiology of this disease.