The extent and pattern of drug resistance among previously treated tuberculosis patients was investigated. Ninety-six patients with a total treatment duration of between 6 and 18 months and still smear and culture positive were examined. Treatment was either continuous or in intermittent blocks. Drug susceptibility tests on strains of tubercle bacilli isolated from the patients were performed against isoniazid, streptomycin, p-aminosalicylic acid, ethambutol and rifampicin by the proportion method using LJ medium without potato starch. A total of 56% of the strains were resistant to one or more of the drugs tested. Resistance to isoniazid (38%) and streptomycin (29%) was most common. A significant finding in the study was the low level of resistance to rifampicin (2%) and ethambutol (3%). A relationship between the incidence of drug resistance and the nature and duration of previous treatment appeared likely since susceptible strains were isolated more often from patients with continuous treatment than from patients on intermittent blocks of long-course regimens. It is therefore suggested that the introduction of better supervision of drug taking and the adoption of continuous short-course regimens on a nationwide level will contribute immensely towards the reduction of the drug resistance problems in Nigeria as well as in other developing countries.