We studied the background of 78 patients with pulmonary nontuberculous mycobacteriosis from 1992 to 1996, and 56 patients (71.8%) were diagnosed as primary infection type and 22 patients (28.2%) as secondary infection type. The former consisted of 17 males and 39 females (mean age +/- SD; 67.4 +/- 12.9 years), and the latter consisted of 13 males and 9 females (mean age +/- SD; 74.0 +/- 7.5 years). Out of all 83 strains, 67 strains (80.7%) were M. avium complex, and out of 36 strains identified either M. avium or M. intracellulare, 34 strains (94.4%) were M. intracellulare and 2 strains (5.6%) were M. avium. Many patients lived in farming areas along the coast, and 35 patients (62.5%) of primary infection type and 11 patients (50%) of secondary infection type were or used to be farmers. Nine patients (16.1%) of primary infection type and 6 patients (27.3%) of secondary infection type had history of gastroduodenal ulcer, while only 3 of all 78 patients had sinusitis. There was a married couple who lived in a same house and 2 sisters who lived apart who were proved to have primary infection type but cross infection was not demonstrated in either case. The development of this disease seems to be related to a genetic susceptibility and environmental factors.