As a result of numerous trials, isoniazid prophylaxis was shown to be effective in preventing tuberculosis in many different populations and under a variety of conditions. However, the duration of the protective effect has been of some concern. In a previous report, the protective effect of isoniazid prophylaxis among Alaskan Eskimos was shown to persist through the fifteenth year after its administration. In this final report, the protective effect is shown to persist for more than 19 years. The magnitude of the effect is related to the amount of isoniazid taken. The results of the study are consistent with the hypothesis that the decrease in risk of tuberculosis produced by isoniazid preventive therapy is lifelong.