The incidence of infections caused by opportunist mycobacteria in Wales during the years 1952--1978 was derived from the records of the Mycobacterium Reference Unit in Cardiff. Mycobacterium kansasil gave rise to 154 pulmonary and two nonpulmonary infections. Most of these patients were middle-aged men, approximately half of whom had pneumoconiosis or occupational exposure to dust. Organisms of the Mycobacterium avium-intracellulare-scrofulaceum complex caused pulmonary disease in 44 subjects and nonpulmonary disease in 20. Those with pulmonary infections had a mean age of 57 years, and the majority of the males in this group were or had been coal miners. Of the nonpulmonary infections, 18 were cases of cervical lymphadenopathy in young children. Other mycobacteria accounted for 18 cases of pulmonary disease (mycobacterium xenopi, 9; Mycobacterium malmoense, 8; Mycobacterium szulgai, 1), but rarely caused nonpulmonary infections. From these figures it was estimated that there are 200 cases of infection with opportunist mycobacteria in Great Britain per year.