To assess the efficacy of unilateral laparoscopic ovarian diathermy in the induction of ovulation in anti-estrogen-resistant polycystic ovary syndrome (PCOS).A prospective randomized study was performed to compare unilateral with bilateral ovarian diathermy.Specialist Reproductive Endocrine Unit.Ten patients with anti-estrogen-resistant PCOS.Randomization to unilateral (4 patients) or bilateral laparoscopic ovarian diathermy (6 patients).Rate and side of ovulation and change in endocrine profiles after ovarian diathermy.Unilateral ovarian diathermy resulted in ovulation from both ovaries. Fifty percent of the patients responded to diathermy and those who responded had a significantly greater fall in serum LH concentrations than those who failed to respond.The mechanism of action of laparoscopic ovarian diathermy is via a correction of disturbed ovarian-pituitary feedback. Hypersecretion of LH appears to be the most significant endocrine disturbance in these patients.