The management of the adult tuberculin reactor who has no other risk factors for the development of active tuberculosis is controversial. We performed a decision analysis to address the issue of whether adults 20 to 80 yr of age should take isoniazid to prevent the development of active disease. The analysis uses a Markov simulation, a regression model to calculate the risk of developing tuberculosis at any given time after the discovery of a positive skin test, and estimates from the literature for other probabilities. The analysis favors withholding INH for all age groups, but by only 4 to 17 days of life expectancy, depending on the age of the patient. Quality adjusting and discounting change neither the preferred strategy nor the closeness of the outcome. Sensitivity analyses also underscore the narrowness of the margin. Our analysis reaches different conclusions from previously published analyses. More important than the actual strategy preferred is the minute difference in expected utilities between administering and withholding INH. We suggest that the individual patient decide whether or not these differences are meaningful.