Basing on a retrospective analysis of 169 cases of relapses of tuberculosis that had been confirmed by bacteriological examinations, we explored possible causes and/or factors favouring a relapse. 6.5% of the mycobacteria cultivated when the relapse was discovered, were resistant to one antibiotic, and 5.3% to two ore more antibiotics. On treating the previous disease, side effects were seen in 31.6%, in 22.3% chemotherapy of the previous disease had been insufficient in respect of duration and/or continuity and/or combination, and in 22.5% the intake of drugs had lacked controlled supervision. In 56% of the relapses there were concomitant diseases (alcoholism in 24.5%, silicosis in 14.9%, diabetes mellitus in 9.7% and gastrointestinal diseases in 5.2%).