Immunotherapy with Mycobacterium w vaccine was attempted in patients with borderline-borderline, borderline lepromatous (BL), or lepromatous leprosy (LL) to determine whether immunization can hasten recovery and reduce treatment time by invigorating cell-mediated immunity. Mycobacterium w, a nonpathogenic, rapidly growing, atypical mycobacterium, shares a number of common B and T cell determinants with Mycobacterium leprae and Mycobacterium tuberculosis. Patients receiving the vaccine had rapid clinical improvement and accelerated bacteriologic clearance. After treatment with vaccine for 2 years, 13 of 31 BL and LL patients were bacteriologically negative as were 5 of 25 controls. Vaccinated patients had one of two distinct histologic features, either an upgrading in the disease spectrum or complete clearance of granuloma. Some 80% of lepromin conversions were in BL and LL patients who received vaccine versus none and 14.3% of BL and LL controls, respectively. Thirteen of 17 vaccinated LL patients were released from treatment after 2 years in contrast to 2 of 15 controls.