During a period of 11 yr (1983-1993) 137 clinical isolates of Mycobacterium simiae obtained from 75 patients were identified in a University hospital in San Antonio, Texas. One hundred twenty-eight isolates (93%) were from a pulmonary source, four (3%) from blood, and five from other sites including skin, urine, lymph node, bone marrow, and brain. Of 62 evaluable patients, six (10%) had definite infection, nine (14%) had probable disease, and 48 (76%) were thought to be colonized. During the last 2 yr of the study (1992 and 1993), M. simiae became the second most frequently isolated nontuberculous mycobacterium at this institution surpassed only by Mycobacterium avium complex. There are limited data about effective treatment for this multidrug-resistant organism. New macrolides, quinolones, ethambutol, clofazimine, and aminoglycosides are promising therapeutic agents.