DNA fingerprinting based on the detection of the insertion sequence IS6110 in Pvull restriction fragments was applied to M. tuberculosis isolates originating in the first microepidemic of multidrug resistant tuberculosis recorded in the Czech Republic. Their disseminators were 21 individuals living in--or roaming between three distant areas. The age of 17 males ranged from 36 to 64 years (average 45 years) and of 4 females aged from 38 to 52 years. The index person was most probably a former male prisoner, aged 49 years, who disseminated multidrug resistant M. tuberculosis over a period of 28 months. In ten of the patients the following risk factors for tuberculosis were found: imprisonment, homelessness, immigration and previous stay in asylum--or in a psychiatric ward. In six cases, M. kansasii infection preceded tuberculosis. Four out of the 21 patients died. The RFLP analysis separated the patients into two distinct groups: group A comprising 14 members of which M. tuberculosis strains were isolated with six IS6110 copies, whereas the isolates of seven individuals of the group B, the RFLP profile displayed highly similar RFLP patterns compared to the isolates of group A, but with two additional IS6110 copies. In one patient, both A and B patterns were found: the first one in a M. tuberculosis strain isolated in 1993 and the second one in the isolate isolated two years later. Both the appearance of pattern B among the isolates of a part of patients and the switch from A to B pattern in one of patients can be plausibly explained by the unstability of DNA genotypes caused by transposition of IS6110 elements.