Based on a study indicating a significant rate of reactivation of tuberculosis in Veterans Administration patients with inactive disease, a cooperative study was initiated to determine the prophylactic effect of isoniazid on the rate of reactivation. A randomized double-blind study was designed, utilizing three regimens, two with isoniazid and one with placebo only. Two consecutive years of taking pills in one of the three regimens was followed by five years of observation. A total of 7,036 patients with inactive disease, some of whom had received prior chemotherapy, were entered into the study. Only 63 reactivations of tuberculosis were found, for a total rate of reactivation of 9/1,000 (less than 1%) over the seven-year period. Although no significant differences in the rate of reactivation were found among any of the regimens, there was a significant reduction in the rate of reactivation among those who had not received any prior chemotherapy and received isoniazid (INH) in this study, compared with those who received placebo only.