Of 109 cases in England and Wales in which mycobacteria of the avian-Battey group were isolated from man 89 were considered genuine infections (including 32 with Mycobacterium avium) and six were doubtful. Almost half the significant cases were lung infections in men in dusty occupations, mostly coal miners with pneumoconiosis, and just over a quarter were non-pulmonary, mostly cervical adenitis in children. The prognosis in lung infections was serious, especially in the presence of pneumoconiosis.