Among 139 children with acquired immunodeficiency syndrome at Children's Hospital of New Jersey, 20 had positive cultures for non-tuberculous mycobacteria. Eighty-five percent had Mycobacterium avium complex isolated and 70% had definite evidence of disseminated disease. Ninety-three percent had CD4 lymphocyte counts less than 100 cells/mm3 and 95% had met acquired immunodeficiency syndrome criteria before the time of first positive culture. Clinical findings included failure to gain weight, anorexia, fever, abdominal pain/tenderness and anemia. The median age at onset of symptoms was 46 months and the median time between onset of symptoms and positive culture was 9 months. Outcome for patients with positive cultures for nontuberculous mycobacteria was poor, with 75% of the children surviving for less than or equal to 10 months. Nontuberculous mycobacteria are increasingly important causes of morbidity and indirect mortality in human immunodeficiency-infected children. Children with severe immunodeficiency are at particular risk. In addition to M. avium complex, other species of nontuberculous mycobacteria may be involved.