During the 2-yr period 1981-83, demographic, clinical, and laboratory information was collected for 5,469 patients from whom nontuberculous mycobacteria (NTM) had been isolated. Among the potential NTM pathogens, isolates of Mycobacterium avium complex were most frequent, followed by M. kansasii, M. fortuitum, M. scrofulaceum, and M. chelonae. Almost 90% of the isolates were obtained from respiratory specimens. Prevalence rates for NTM disease, as calculated by a diagnostic algorithm, were highest for M. avium complex (1.3/10(5)), M. fortuitum-M. chelonae (0.2/10(5)). The data suggest a changing epidemiologic picture of NTM disease due perhaps to the decreasing incidence of tuberculosis, the increasing prevalence of chronic lung disease, and increased culturing of diagnostic specimens, as well as possibly a change in the ecology of these organisms.