The records of 81 patients with isolations of Mycobacterium kansasii in Western Australia over 26 years have been reviewed. Thirty-nine individuals with 40 episodes of infection were considered to have disease due to M. kansasii: 36 in the lungs (90%), two in the joints (5%), and one each in the skin (2.5%) and the lymph nodes (2.5%). The average incidence rate per 100,000 per year was 0.14 for the state with the highest in the Geraldton Mid-west region (0.57). The male:female ratio was 4:1 and the mean age 50 years (SD 14.5). For pulmonary disease the male:female ratio was 3.5:1, the mean age 53 years (SD 11), 42% being miners or ex-miners and 91% smokers or ex-smokers. Almost 97% of those infected were Caucasian and none were Aboriginal. Drug treatment was highly successful, particularly with the rifampicin/ethambutol combination, where 12 months appeared to be adequate. There are differences in the demography of the patients having pulmonary disease compared to the 42 with insignificant isolations.