Sir.—The optimal treatment of patients with histiocytosis X remains uncertain. Preliminary clinical studies1-6 have demonstrated that a thymic hormone preparation may be a useful therapy for this disease. The Food and Drug Administration is sponsoring a clinical trial of Suppressin A, one such thymic hormone drug. The purpose of this prospective, randomized, double-blind, placebo-controlled study is to determine if Suppressin A is biologically active in histiocytosis X. I am the principal investigator of this study and am writing to you at this time to solicit patient referrals. The study includes both children and adults with histiocytosis X and patients can be either newly diagnosed or recently relapsed. Please write or call me collect ( 638-4182) for further information. Thank you very much for your cooperation in this regard.