To investigate the prevalence of Mycobacterium tuberculosis DNA in granulomatous tissues from patients with sarcoidosis and from controls matched for age, sex, and tissue by using the polymerase chain reaction. Single blind control trial. 16 patients with sarcoidosis who had undergone diagnostic biopsy of lung, skin, or lymph node and 16 patients with squamous cell carcinoma or Hodgkin's disease to act as controls. In addition, four lung specimens infected with M tuberculosis were included as positive controls. M tuberculosis DNA was present in sarcoid tissues containing granulomas from seven of the 16 patients and one of the 16 matched controls. Two of the four specimens known to be infected with M tuberculosis were positive in the controlled experiment. These figures suggest that M tuberculosis DNA is detected as readily in patients with sarcoidosis as in patients with frankly tuberculous tissues and imply that M tuberculosis may be linked to the cause of sarcoidosis.