One hundred fifty-six previously untreated Indian patients with visceral leishmaniasis were treated with pentavalent antimony (Sb) alone for 30 days (group A), Sb plus interferon-gamma (IFN-gamma) for 30 days (group B), or Sb plus IFN-gamma for 15 days (group C). The purpose was to show that IFN-gamma would increase the response to 30 days of Sb treatment and that short-course (15 days) combination therapy was as effective as 30 days of Sb alone. Six months after treatment, 36% of group A, 49% of group B, and 42% of group C patients were designated as definitively cured. The success rates for long-term responses to Sb alone (36%) and Sb plus IFN-gamma (49%) were unexpectedly low, and responses in groups A, B, and C were not significantly different. These results suggest that the beneficial effects of adjunctive IFN-gamma in visceral leishmaniasis may be limited in regions where this disseminated intracellular infection shows high-level resistance to Sb.