The decision to prescribe or withhold isoniazid (INH) preventive therapy for low-risk tuberculin reactors has been highly controversial, primarily due to isoniazid's possible hepatotoxic effects. Previous analyses have explored the INH decision only from the perspective of patient age, recognizing that the risks of INH-induced hepatotoxicity are age related. Decision analyses presented in this paper assess the impact of gender and ethnic group, as well as age, on the INH decision. Results for low-risk patients favor prescribing INH preventive therapy for all 20-yr-olds, all 35-yr-olds except black women, and no 50-yr-olds, projecting life expectancy benefits that range from 3 to 19 days. A comparison set of analyses performed for high-risk patients favors prescribing INH for all groups except 50-yr-old black women. These findings suggest that ethnicity, gender, and age should be considered when making the decision to prescribe or withhold INH preventive therapy.