Disseminated mycobacterial infection, often due to Mycobacterium avium complex, occurs frequently in patients with AIDS. More recently, a newly identified mycobacterium, Mycobacterium genavense, has been repeatedly isolated from AIDS patients. Mycobacterium genavense has unusual fastidious growth requirements and shows poor and variable growth in vitro. Molecular biology techniques are necessary for accurate diagnosis of infection and have established Mycobacterium genavense to be a definite cause of disseminated mycobacterial infection in immunosuppressed patients. The clinical manifestations of infection caused by Mycobacterium genavense are similar to those of infection caused by Mycobacterium avium complex organisms. This similarity in clinical presentation and inherent difficulties in applying standard, culture-based techniques for detection and identification of mycobacteria have probably led to underestimation of the prevalence of disseminated Mycobacterium genavense infection in patients with AIDS.