Recommendations regarding how long isoniazid should be taken to prevent tuberculosis have gradually shifted from 12 to 6 months of treatment. These recommendations and the relevant evidence from controlled trials on duration of treatment are reviewed. The conclusions for immunocompetent adults are: 1) 6 months of preventive treatment does not give optimal protection; 2) more than 12 months of preventive treatment is not necessary; 3) 9-10 months appears to be the optimal duration; and 4) total duration of preventive treatment may be more important than its continuity.