During a 10 year period 49 patients were recorded as having pulmonary infection caused by opportunist mycobacteria. Six different species were identified of which M. kansasii (65%) and M. avium-intracellulare (20%) were the most common. Cough and sputum (82%) or haemoptysis (26%) were frequent symptoms on presentation and over two thirds of patients had pre-existing respiratory disease. Chest radiographs showed predominantly apical disease with the right apex (44%) being twice as commonly affected as the left (22%). In 30% the radiograph showed bilateral disease. Clinicians had an individual approach to treatment with no uniform pattern of drug prescribing. A majority of patients (59%) received rifampicin for at least 9 months and those patients with M. kansasii infection responded well with no bacteriological relapses in 20 patients followed for a mean period of 3.9 years. M. avium-intracellulare, M. malmoense and M. xenopi were less responsive to treatment and in four patients receiving chemotherapy death was attributed to mycobacterial infection.