The purpose of this study was to determine whether any of the Mycobacterium kansasii cases were the consequences of primary lung malignancy. The records and chest x-ray films of 295 patients with M kansasii pulmonary infection were reviewed. The infection was found to complicate the primary lung neoplasm in four cases. Three patients had had treatment for malignancy: one patient with small cell carcinoma received chemotherapy, steroids and radiation; one with adenocarcinoma underwent a lobectomy and radiation; and the third patient had a lobectomy and radiation for malignant fibrohistiocytoma. The fourth patient developed the infection three years after lung malignancy manifested itself, which was only a few months before the clinical evidence of distant metastasis with adenocarcinoma was detected. We suggest that this infection be considered in patients from M kansasii endemic areas, especially after they have received radiation treatment for lung malignancy. This association has never been described before.