The purpose of this study was to determine the polymorphism of insertion segment 6110 (IS6110) in strains of Mycobacterium tuberculosis isolated from Colombian patients as well as the current status of resistance to antituberculosis drugs in the department of Quindío, Colombia. To this end, a prospective study was performed with a consecutive sample of 59 patients who sought care at local health centers and hospitals in rural and urban areas of Quindío from March to July 1993. The patients in the sample had symptomatic pulmonary tuberculosis confirmed by bacteriologic inspection of sputum, with and without a history of treatment, and were participants in the Tuberculosis Control Program of the Sectional Health Institute of Quindío in Armenia, Colombia. Sputum cultures and drug sensitivity tests were done. Later, restriction fragment length polymorphisms (RFLP) of IS6110 were analyzed in accordance with the protocols of van Soolingen et al. (1992). Cases were classified by treatment history, applying the criteria of WHO (1991). The results showed 44 cultures positive for M. tuberculosis and one positive for M. africanum. Primary drug resistance was found in 4 of 42 cultures, or 9.5% (CI 95%: 0.6 to 18); 4.8% were resistant to isoniazid (INH) and 4.8% to isoniazid and streptomycin (INH-SM). Acquired resistance was found in two of three cultures, or 66% (to isoniazid, rifampicin, and streptomycin [INH-RM-SM] and to isoniazid, ethambutol, rifampicin, and streptomycin [INH-EMB-RM-SM]). In 27 strains submitted to RFLP analysis, the number of copies of IS6110 varied from 6 to 17. Similarity coefficients revealed five distinct groups.(ABSTRACT TRUNCATED AT 250 WORDS)