To determine the efficacy of evaluating persons (associates) in close contact to children with significant tuberculin reactions, we prospectively evaluated 831 associates of 297 children younger than eight years who had significant (greater than or equal to 10 mm) tuberculin reactions. Eighty-seven per cent of the index reactors were foreign-born, as were 84 per cent of the associates. All associates were evaluated by tuberculin skin testing; chest roentgenograms and sputum cultures were obtained if indicated. Four hundred sixty-one (55 per cent) of the associates had significant tuberculin reactions, and 15 had current tuberculosis. However, only three of these cases were newly discovered (total case rate: 1.81/100, new case rate: 0.36/100). Two of the three new cases were detected in the associates of children younger than three years of age. In addition, 338 candidates for isoniazid (INH) preventive therapy were found. We conclude that although the yield of new cases was low, the evaluation of associates did provide a convenient, high yield method of identifying candidates for INH preventive therapy. Moreover, it served as a useful mechanism for monitoring the adequacy of other case-finding activities.