We studied reactivity to tuberculin skin testing in nearly all nursing home residents in Arkansas. Only 12 per cent of the 12,196 newly admitted residents were tuberculin positive, as compared with 20.8 per cent of the 13,441 residents who were first tested more than a month (mean, 30 months) after admission. The proportion of persons who were positive on initial testing varied greatly with the time spent in the home before testing. Those who were not reactive on initial testing had a 5 per cent rate of conversion for each year spent in a home with a known recent infectious case (within three years) and a 3.5 per cent rate for each year in a home with no recognized recent case. Active tuberculosis developed in only 1 of 534 persons with positive tuberculin tests or previous reactions who were treated with isoniazid, but in 79 (2.4 per cent) of 3270 persons who were not (P less than 0.001). The disease developed in only 1 (0.16 per cent) of 605 persons whose tests converted to positive and who were treated with isoniazid, as compared with 45 (5.9 per cent) of 757 whose tests converted but who were not treated (P less than 0.001). We conclude that new infection with tuberculosis is an important risk for nursing home patients and that greater care should be taken to detect and treat new infections before the disease develops and the infection spreads.