Mycobacteriosis was found in 59 patients with malignant disease in a five-year period from 1968 to 1973. Thirty patients (51%) had mycobacteriosis that was caused by atypical mycobacteria. The most frequent organisms were Mycobacterium kansasii and M fortuitum. The most frequent tumors associated with mycobacteriosis were squamous cell carcinoma of the head and neck, testicular carcinoma, and lung carcinoma. The only predisposing factor was treatment with cancer chemotherapy. Mortality due to mycobacterial infections was most frequently associated with those infections caused by M tuberculosis. Although mycobacteriosis is relatively uncommon in cancer patients, if it is present, atypical mycobacteria should be looked for because, in these patients, these organisms cause more than 50% of mycobacteriosis.