Background:Amenorrhea is a common adverse effect of treatment with antipsychotic medications that influences both fertility and adherence to medication regimens. Most research suggests that medication-induced prolactinemia is the main cause of amenorrhea but few prospective studies have assessed this hypothesis. Aim:Identify risk factors for amenorrhea following treatment with antipsychotic medication. Methods:The study used a prospective, nested case-control design. First-episode, drug na?ve female patients with schizophrenia who were in the middle of their menstrual cycle at the time of admission were enrolled. Serum levels of six reproductive hormones were assessed before and after a 12-week course of treatment with risperidone:progesterone, estradiol, prolactin, follicular stimulating hormone, luteinizing hormone, and testosterone. The hormone levels of 31 patients who had no menstruation during the entire 12 weeks of treatment (the amenorrhea group) were compared to those of 31 age-matched subjects who had normal menstrual periods over the 12 weeks of treatment (the control group). Results:We found a dramatic 4-fold increase in prolactin levels in women of childbearing age treated with risperidone, but the pretreatment and posttreatment levels of prolactin were not different between patients who did and did not develop amenorrhea with treatment. However, there were significantly lower pretreatment levels of estradiol and progesterone in patients who subsequently developed amenorrhea with risperidone treatment than in patients who did not develop amenorrhea. A conditional logistic regression analysis found that pretreatment levels of estradiol remained significantly associated with the development of amenorrhea during treatment even when adjusting for the pretreatment levels of the other five reproductive hormones assessed. Conclusion:These findings do not support the suggestion that amenorrhea associated with the use of antipsychotic medication is the result of hyperprolactinemia. If our finding of the predictive power of pretreatment levels of estradiol is confirmed in larger studies, this information would be of use to clinicians in selecting antipsychotic medications for female patients with schizophrenia;patients at highest risk of developing amenorrhea could be preferentially treated with the medications that are at lowest risk of inducing amenorrhea.