The apparent increase in the incidence of opportunist mycobacterial pulmonary infection is assessed. The clinical, bacteriological and radiological characteristics of 34 cases of such infections seen over a period of ten years in Sheffield are reviewed. The typical radiological appearances consisting of cavities, fibrosis and opacities are described and illustrated. There has been in the past conflicting reports on the specificity of such radiological features; it is concluded that although such appearances are not pathognomonic, their presence should arouse suspicion of such an infection and this should be confirmed or refuted by bacteriological investigations.