Visceral leishmaniasis is infrequently reported in immunocompromised hosts; hence, the clinical manifestations and outcome of the disease in these patients are unknown. In a series of 10 patients with visceral leishmaniasis complicating renal transplantation (three), hematologic neoplasms (two), systemic lupus erythematosus (two), or infection with human immunodeficiency virus (three), typical hallmarks of kalaazar such as enlargement of spleen or hyperglobulinemia were absent in three and six patients, respectively. Extensive visceral involvement was noted by biopsies or autopsies in four patients. Diagnosis was made during evaluation for fever of unknown origin. Myriads of amastigotes were seen in bone marrow smears. Measurement of antibodies against Leishmania donovani was positive in each patient tested. Ultimately, three patients died, and chronic infections refractory to treatment developed in two other patients. Visceral leishmaniasis is a potentially fatal infection in immunocompromised hosts. Current antiparasitic therapy frequently fails to eradicate L. donovani from infected tissues.