OBJECTIVE A group of patients with pulmonary tuberculosis (PT) who received and correctly completed antituberculous therapy were studied to determine the incidence and significance of positive acid-fast bacilli (AFB) in sputum smears at the end of this treatment. METHODS Retrospective chart review of persons with bacteriologically proved PT. METHODS Of 1,052 patients diagnosed as having PT between 1988 and 1992, 453 who fulfilled the following criteria were included in the study: (1) diagnosis established by positive AFB and positive culture in sputum smears; (2) no previous antituberculous treatment had been received; (3) HIV serologic test results were negative; (4) treatment was correctly completed; (5) they were followed up throughout the period of treatment; and (6) expectoration was still present at the end of treatment and at least two spontaneous sputum samples could be obtained. RESULTS Positive AFB of sputum smears were found at the end of treatment in 10 (2.2%) of the 453 patients studied. Five patients had only one positive smear, and the other five had more than one. Of these ten cases, sputum culture was negative in eight, which were considered to be unviable bacilli, and positive for nontuberculous mycobacteria in two. Clinical symptoms or worsening on chest radiograph were observed only in one patient with unviable bacilli, but they were caused by a concomitant nonspecific respiratory tract infection. CONCLUSIONS Positive AFB smear results at the end of completed treatment regimens analyzed in this study have occurred because of unviable bacilli and nontuberculous mycobacteria colonization. The presence of more than one positive smear seems not to increase the probability of treatment failure and is more frequently due to nontuberculous mycobacteria. Results of culture can thus be awaited without the need to prolong or modify antituberculous therapy.