Data supplied by tuberculosis reference laboratories in Australia indicate that from mid-1985 to the end of 1988, 202 patients with the acquired immunodeficiency syndrome (AIDS) were diagnosed as having concomitant infection caused by organisms of the Mycobacterium avium complex. This figure represents 17.3% of all AIDS cases diagnosed. Serotyping showed that the majority of isolates were serovars of M. avium, namely serovars 1, 4 and 8. There is evidence of geographic differences in the dominant serovar. Of 45 patients with isolates from more than one site of the body, 17 (38%) were infected with at least two different serovars. The findings show the importance of disseminated atypical mycobacteriosis in patients with AIDS, and they provide data for use in defining the epidemiology and natural history of such infections.