An open trial of liothyronine 20μg twice daily was carried out in 26 women, selected from our general medical and endocrine clinics, with severe or moderately severe acne. They were on no other treatment for acne during the trial. Endocrine tests showed mild hyperprolactinaemia which was reduced, but not significantly, during treatment. Response scores were assessed at intervals of 3 months for a total period of 1 year. Little change occurred in the first two months, but the score was significant at 3 months (t-test p<0.001) and progressed most between 6 and 9 months. The explanation for the response is uncertain and requires further work.