A study was undertaken in 18 leprosy patients an d 31 controls to find ou t if Lamprene can prevent a flare-up of the infection with My co. leprae during 10ng-term steroid therapy. All patients were either BL or LL; some were given daily Lamprene 100 mg and prednisone 10 mg because of persistent reactions, others twice weekly Lamprene 100 mg an d daily prednisone 10 mg for the same re ason. The duration of treatment varied fram 6 to 23 months. The control patients were given Lamprene only, either daily 100 mg or twice weekly 100 mg. Their reactions we re less severe an d there fore they did not re quire prednisone. It was found that in both groups (L amprene plus prednisone and Lampr�ne only) there was a satisfactory reduction of the MI (M orphological Index) to zero ar less than 1 %. But as regards the BI (B acteriological Index) the patients on Lamprene plus prednisone did clearly better than the patients on Lamprene only. It thus appears that long-term steroid therapy has no adverse effect on the BI and MI of lepromatous patients, provided that they are treated with Lamprene at the same time.