Study of 49,467 persons over age 50 in Arkansas nursing homes afforded insight into the significance of the tuberculin skin test in the elderly. Whereas only 15% to 20% of persons showed a significant (10 mm or more) reaction to tuberculin on admission, 2% to 3% of these developed tuberculosis. Persons having no reactions comprised two subsets: a small group who died at an increased rate and were probably anergic, and a larger group who survived as well as persons who had reactions. Minor increases in reaction size with repeated testing appeared to be due to immunologic recall. However, conversions of 12 mm or more from a documented negative result indicated spread of infection. When not treated preventively, 7.6% (women) to 12.7% (men) of definite converters developed tuberculosis. The increase in number of persons showing positive reactions after entry may have been due to rapid demise of the anergic subset, improvement in nutrition of survivors, or unsuspected spread of tuberculous infection.