Three-hundred fifty consecutively autopsied AIDS patients in Milan, Italy, were studied to determine the frequencies of clinical and postmortem diagnoses of mycobacterial diseases, to evaluate the clinical histories of those patients with mycobacterial diseases, and to investigate the reasons for nondiagnosis of mycobacterial diseases during life. Seventy-eight patients (22.3%) had mycobacterial diseases. In 64 cases (18.3%) the diagnosis was made antemortem and in 50 (14.2%) at autopsy; there were 36 cases of concordant clinical and postmortem diagnoses. Nontuberculous mycobacterioses (NTM) were diagnosed in 41 patients (20 clinical/postmortem diagnoses, 11 clinical diagnoses, and 10 postmortem diagnoses), extrapulmonary tuberculosis (TB) in 19 patients (7 clinical/postmorten, 8 clinical, 4 postmortem), and pulmonary TB in 18 patients (9 clinical/postmorten, 9 clinical). Patients with a clinical diagnosis of mycobacteriosis but with no pathological evidence of disease at autopsy were considered to have recovered on the basis of negative culture findings and prolonged antimycobacterial treatment. In Italian patients with AIDS, NTM occurs less frequently and TB more frequently than in American AIDS patients. At least some of the patients reported in this study seemed to have recovered from mycobacterial disease after prolonged treatment. The lack of diagnosis during life can be attributed to aspecific symptoms, a rapidly terminal course, and the presence of concomitant opportunistic diseases.