In 67 female children receiving therapeutic prednisone nuclear sex determinations were carried out before and during therapy. In all 67 patients a reduction of the sex-chromatin incidence was noted. In 12 patients who were studied after therapy was stopped the incidence of sex chromatin was found to return to the normal female range. In six patients in whom the smears were studied at intervals of four days for 32 days during therapy the sex-chromatin incidence showed a rapid initial fall noticed within the first four days, and then minor fluctuations without further fall during the continuation of therapy. © 1966, British Medical Journal Publishing Group. All rights reserved.