Records of 244 patients with Mycobacterium kansasii isolated from their sputum were reviewed. Of the 244 patients, 82 failed to meet study criteria and were excluded. Response to treatment and posttreatment follow-up was evaluated in 162 patients. Overall, 135 patients (83%) achieved negative sputum cultures within six months. Among 32 patients whose drug regimen included rifampin, all 32 (100%) had negative cultures at the fourth and fifth months, but two relapsed during the sixth month, with cultures resistant to 1.0 and 25 micrograms of rifampin/ml. Among 130 patients whose regimen did not include rifampin, negative sputum cultures were achieved by the sixth month in 101 (80%). In these patients, no significant influence could be attributed to (1) the use of three-drug vs. two-drug regimens; (2) the in vitro susceptibility of pretreatment cultures to isoniazid; (3) the coexistence of obstructive airway disease; or (4) the early use of pulmonary resection. The susceptibility of pretreatment cultures to streptomycin in those who received this drug may have influenced outcome. The late follow-up showed a cumulative relapse rate of 13% over five years in those who had achieved negative cultures initially, and the relapse rate did not appear to be influenced by pulmonary resection as part of the treatment.