In a double-blind, placebo controlled study, conjugated estrogens (CE) (0.625 mg/day) were added to a fixed dosage of haloperidol (5 mg daily). Forty-four female inpatients with acute schizophrenia were included in the study and randomized to one of the groups; 40 patients completed the trial. They were followed for 28 days and evaluated periodically with the BPRS, Negative Symptoms Rating Scale, Simpson Angus Extrapyramidal Rating Scale and UKU rating scale. Hormonal concentrations (estradiol, estrone, progesterone, FSH, LH and prolactine) were measured at baseline and weekly throughout the trial. Both groups showed similar clinical improvement during the evaluation, although there was a trend for the CE group to show a better improvement than the placebo group (p < 0.10). Side effects and the use of anticholinergics were similar in both groups. Conjugated estrogens caused elevation only of estrone levels in the CE group; estradiol and prolactin showed a similar profile for both groups. Our negative findings regarding the antipsychotic effect of conjugated estrogens does not preclude, however, a possible efficacy of other estrogens, such as 17-beta-estradiol, in schizophrenia.