In a retrospective study to evaluate the efficacy of isoniazid (INH) for the prevention of tuberculosis, we studied 1760 patients with hematological malignancies over a twenty-year period (1970-1989). 759 patients received oral INH, most of all received 400 mg per day. Only one (0.1%) of the patients receiving INH developed tuberculosis, whereas nine (0.9%) of the 1001 patients who did not receive INH developed tuberculosis (p less than 0.05). We found that INH was very effective in the prevention of tuberculosis in patients with hematological malignancies, and was well tolerated.