Over the period 1968-1979, Mycobacterium kansasii was isolated from a total of 297 persons residing in the district of Karviná, North-Moravian Region, Czechoslovakia. Repeated M. kansasii isolations were recorded in 153 patients with clinical symptomatology of chronic respiratory disorders, single isolations in another 144 persons (121 isolations from sputum and/or laryngeal swabs, 22 from urine, and one from the aspirate of a lymph node with tuberculoid granuloma). The incidence of infection with repeated M. kansasii isolation increased from 1.1 to 11.8 per 100 000 population. The ratio of these infections to the number of newly notified cases of pulmonary tuberculosis increased from 3.7 to 33.7%. The average age of patients was 46 years, 84% of all notified cases occurred by 54 years of age and morbidity was highest in the age group 45-54 years and reached 143/100 000 population. The proportion of male patients was 88% and 56% were heavy industry workers occupationally exposed to high dust concentrations.