DURING the past year, isonicotinic acid hydrazide (isoniazid) has been employed widely in the treatment of various types of systemic tuberculosis.1 Because of the favorable nature of preliminary studies, a trial of the drug seemed warranted in the treatment of tuberculosis involving the skin. This report summarizes observations on 7 patients with lupus vulgaris, 2 with scrofuloderma, 11 with erythema induratum (Bazin's disease), and 3 with papulonecrotic tuberculid. Two patients are reported on in detail. Four patients with sarcoidosis and one with the so-called rosacea-like tuberculid have been included. Leprosy has many clinical, pathological, and immunological similarities to tuberculosis; accordingly, a trial of isoniazid in the treatment of this disease was deemed important. Seven patients with leprosy were treated, three with the lepromatous, three with the tuberculoid, and one with the indeterminate form of the disease. Finally, a group of patients with miscellaneous skin diseases, including eight