Mycobacterium xenopi comprised 56% of all non-tuberculous mycobacteria isolated in the Brompton Hospital laboratory during six years. M xenopi alone was cultured from the sputum of 23 patients, whose clinical and bacteriological features are reviewed. Pulmonary disease was considered to be due to the organism present in 15 of these patients on the basis of strict criteria for a causal relationship. Radiographs of all these patients were consistent with mycobacterial disease, showing pulmonary cavitation in 11, apical shadowing in three, and upper lobe fibrosis in one. In nine patients the disease was chronic, progressing very slowly with longstanding respiratory symptoms and extensive radiographic abnormalities developing over many years (mean 19 years). In six patients the disease was subacute, being of recent onset with radiographic shadowing of limited extent. M xenopi was isolated as a saprophyte from four patients who had lung disease of known cause and single isolates were obtained from four patients who had no lung disease attributable to this organism. In vitro drug sensitivity tests showed the sputum cultures of 17 of 22 patients to be sensitive to two or more of the drugs rifampicin, isoniazid, and ethambutol. Response to chemotherapy was unpredictable and did not always correlate with the results of sensitivity tests. Of 11 patients treated with at least two drugs to which their organisms were sensitive, two remained sputum positive, one relapsed after 18 months of chemotherapy, and eight have remained sputum negative during a minimum of three years' follow-up.