Fourteen wild strains and 14 relapse or treatment failure isolates of Mycobacterium kansasii were tested and found to be highly susceptible to sulfamethoxazole (SMX), with 26 of 28 isolates having minimal inhibitory concentrations (MIC) of less than or equal to 4 micrograms/ml), using a broth microdilution method. Treatment failure isolates frequently exhibited resistance to rifampin (RMP) (greater than 2 micrograms/ml), isoniazid (INH) (greater than 4 micrograms/ml), and ethambutol (EMB) (greater than 4 micrograms/ml) not seen among the wild strain isolates. Eight patients with cavitary disease caused by RMP-resistant M. kansasii were treated with SMX-containing regimens that also included high dose INH (900 mg), EMB (25 mg/kg), and an aminoglycoside (either streptomycin or amikacin). Patients were treated initially in the hospital for 4 to 10 wk. In 7 of the 8 patients, sputum cultures became negative in a mean of 10 wk (range, 7 to 14 wk). Acquired drug resistance to INH, RMP, and EMB can be demonstrated in M. kansasii, and SMX in combination with other agents chosen on the basis of MIC determinations are effective in the treatment of disease caused by RMP-resistant M. kansasii.