An epidemic of tuberculosis among elderly residents of a nursing home was caused by the presence of a highly infectious patient (sputum smear positive) for at least 12 months. Forty-nine (30%) of 161 previously tuberculin-negative residents (mean age, 73.5 years) became infected, and eight (17%) developed progressive primary tuberculosis, including one who died. Also, 21 (15%) of 138 tuberculin-negative employees were infected, of whom one (5%) developed clinical tuberculosis. The epidemic was finally stopped by giving preventive treatment with isoniazid to all converters, despite their advanced ages. Only three of 39 patients so treated manifested toxicity that subsided on withdrawal of isoniazid. The fraction of elderly persons harboring a dormant tuberculous infection today is smaller than generally thought. If one of this group develops active tuberculosis, however, it may endanger 80% to 90% of fellow residents and employees. Tuberculin reactors showed a significant protection against developing tuberculosis from exogenous reinfection.