A man with AIDS and M. kansasii lung infection received rifampicin and isoniazid for 9 months, combined with ethambutol for four months. The treatment was effective with sputum culture negativation, but relapse occurred. The minimal inhibitory concentration of rifampicin for the M. kansasii strain was respectively 0.2 microgram/ml at the onset and 128 micrograms/ml after the treatment, giving evidence of acquired resistance. A new treatment was initiated but is was ineffective.