Disseminated infection by Mycobacterium avium-intracellulare is almost exclusively produced in individuals with HIV infection. The incidence of this infection in Spain is unknown.The clinical and microbiologic registries of 30 patients with AIDS and disseminated infection by Mycobacterium avium are reviewed.Twenty-three percent of the patients with AIDS had, at some time in their evolution, disseminated infection by M. avium. The clinical picture included prolonged fever, digestive symptoms, weight loss and appearance of lymph node enlargement. This infection appeared in patients with severe alteration of cellular immunity (mean CD4 lymphocytes: 0.19 x 10(9)/l). Although medium term prognosis was bad the causes of death of the patients were other opportunistic diseases related with the immunodeficiency.Infection by Mycobacterium avium is frequent among the population of individuals with HIV infection. With the appearance of prolonged fever in a patient with HIV infection and CD4 lymphocyte count lower than 0.2 x 10(9)/1 appropriate microbiologic studies including blood cultures for mycobacteria should be initiated.