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Previous studies have found Mycobacterium leprae in nasal swabs from leprosy patients, their contacts, and persons living in endemic areas. It might be expected that M. tuberculosis would be present on nasal mucosa of pulmonary tuberculosis patients, but whether they can be detected in patients or contacts is unknown. We used the polymerase chain reaction (PCR) technique on nasal swabs from tuberculosis patients, contacts of tuberculosis patients, leprosy patients, and London controls to look for both M. tuberculosis and M. leprae. Swabs dipped in sputum specimens from smear-positive patients were used as positive controls. The PCRs were conducted in two independent laboratories. M. tuberculosis was detected in nasal swabs from 6/16 smear-positive tuberculosis patients and from 1/10 household contacts by one of the laboratories. All of the sputum swabs were positive for M. tuberculosis, and all of the London controls were negative. M. leprae were found in nasal swabs from 2/5 leprosy patients, but one laboratory also reported M. leprae in swabs from 4/21 tuberculosis patients and from one sputum specimen. The results show that M. tuberculosis can be found in the noses of some tuberculosis patients, and suggest that the bacilli also may be detected in some household contacts. The comparisons with M. leprae and between the two laboratories give further insights into the sensitivity and specificity of the technique.