A cluster of cases of tuberculosis among five patients receiving treatment for renal failure was investigated. Insertion sequence (IS6110) fingerprinting and antibiotic resistance profiling of the Mycobacterium tuberculosis isolates from four of the patients (A-D), who had been on the same ward, showed that three of these cases (A-C) were related, but that the fourth (D) was distinct. An isolate from the fifth patient (E), who had been on a separate ward, was indistinguishable from the outbreak strain by IS6110 profile. However, the isolate from patient E and a second isolate from patient A differed from the previous strains in being rifampicin resistant. Sequence analysis of the rpo B genes of the two rifampicin-resistant strains demonstrated the presence of different mutations, showing that they had evolved independently from the same source strain. IS6110 and rpo B gene analyses are invaluable for the accurate investigation of outbreaks of multidrug resistant tuberculosis.