Objectives. To evaluate the DNA polymorphism among Mycobacterium tuberculosis (MTB) strains isolated from new smear positive tuberculosis (TB) patients residing in Tehran capital city of Iran, during the year 2001.

Methods. IS6110-restriction fragment length polymorphism (RFLP) and spoligotyping analyses were performed on 129 M. tuberculosis strains. Additional patient's information was collected for further epidemiological analyses. Patients whose isolates had identical RFLP and spoligotyping patterns were considered a cluster.

Results. The results show that the IS6110 were polymorphic and the strains with 8 or 9 IS6110 copy number were more frequently defected (42%). Out of 129 available isolates, 56 (43%) belonged to clusters and 72 (57%) did not. The risk factors like age, sex, family history or close contact and intravenous drug abuse were associated with clustering. Whereas, unemployment (61%) and poor living conditions (83%) contributed to diseases development in both groups. Spoligotyping of M. tuberculosis strains resulted in 46 different patterns, out of which 38 patterns were unique and reported for the first time. We found one M. tuberculosis strains with a pattern characteristic of the Beijing family.

Conclusion. In the studied time period both reactivation (57%) and recent transmission (43%) were contributing to annual new TB cases in Tehran.