To study the participation of the private sector in detection and diagnosis of tuberculosis, all suspected cases referring to 4 private laboratories in Tehran for acid-fast bacillus examination during 2002–03 were documented. Of 9037 cases enrolled in the study, 637 had positive examination results (7.1%). Of these 531 (5.9%) cases were direct smear-positive and 489 (5.4%) were culture-positive. Data from the Iranian Ministry of Health showed 9479 cases referred to government laboratories in Tehran urban area during the study period, 208 (2.2%) of which were positive. This larger than expected proportion of TB patients who are detected and managed by the private sector indicates that much closer cooperation is needed between the public and private sectors.