Twenty women with the diagnosis of premenstrual syndrome (PMS) participated in a double-blind, placebo-controlled, crossover study to evaluate the efficacy of naltrexone, an oral opiate antagonist. This study was designed to test the hypothesis that inhibition of opiate withdrawal would aid in the treatment of PMS. The subjects received either placebo or naltrexone from days 9-18 of the cycle for three consecutive cycles. The mean scores of the three day-25 Menstrual Distress Questionnaires of 16 patients on naltrexone were compared with the mean scores of the same patients on placebo. Scores were at least ten points lower on naltrexone in 11 patients and at least ten points higher on naltrexone in two patients. Score changes of less than ten points were noted in the other three patients. The mean scores dropped 28 points on naltrexone (P = .016). The general acceptability of naltrexone was good, with side effects including nausea, decreased appetite, and dizziness. These results suggest that naltrexone alleviates many PMS symptoms and may be an effective treatment for this syndrome.