Six cases of pulmonary M. kansasii infection were studied. The ages of the patients ranged from 26 to 51, with a mean of 39.6 years. All the patients were male. None had any underlying systemic or lung diseases. On chest X-ray and CT scan, the majority of cases showed a solitary thin-walled cavitary lesion with little satellite or scattered lesions, which were predominantly located in S1 or S2. The strains of M. kansasii isolated from the 6 patients showed a certain pattern of antibiotic sensitivity being highly sensitive to TH, CS, EB and RFP. In 5 out of the 6 patients, chemotherapy with RFP combined with 2 or 3 other antituberculous drugs for 12 months was successful. In another patient, chemotherapy with RFP, EB and INH for 12 months was unsuccessful and surgical resection was required. Comparison of patients with M. kansasii (n = 6), M. tuberculosis (n = 112) and M. avium complex infection (n = 51) revealed that the former two had some common clinical features: predominance in males, younger age than the patients with M. avium complex infection, predominant involvement of S1, S2 and S6, and involving a single rather than multiple lung lobes. Even in cases clinically suspected of having M. tuberculosis infection, bacteriological examination should be carried out routinely to rule out M. kansasii infection.