Three patients with Mycobacterium simiae infection were seen during a 5-yr period. One patient with chronic infiltrative pulmonary disease had the diagnosis established by open lung biopsy. Two patients with normal chest roentgenograms had disseminated infection, one having had elective surgery for a nonfunctioning kidney. Caseating granulomata were discovered in kidney tissue sections. Both M. kansasii and M. simiae were subsequently recovered from bone marrow and urine cultures. The other patient had vertebral osteomyelitis, and M. simiae was isolated from tissue obtained by needle biopsy. To our knowledge, disseminated M. simiae infection has not been previously described.