An attempt has been made to reconstitute impaired cell-mediated immunity in 1 patient with indeterminate, 4 patients with borderline and 2 patients with polar lepromatous leprosy by grafting three thymus glands obtained from human fetuses of 14-19 weeks gestation. Most of these patients had severe ulcerative erythema nodosum leprosum (ENL) and were intolerant to dapsone. After thymus transplantation these patients were followed for 1½ years. During this period, all conventional chemotherapy had been withdrawn. In most cases, there was dramatic improvement of the clinical condition, resolution of skin lesions, subsidence of ENL, clearance of bacteria from skin and reconstitution of several immunologic deficits; but late lepromin reactivity returned in none, which indicated permanent lose of resistance to Mycobacterium leprae.