The tuberculin reaction following the intradermal injection of PPD appears 48-72 hours after injection. The positivity is shown by an > 5 mm area of induration of the skin. Tuberculin reaction is an invaluable instrument of epidemiologic investigation. Clinically, the value of tuberculin test, though remarkable, is limited by the fact that its positivity is not necessarily a sign of active tuberculosis. The three control strategies of tuberculosis are: prompt identification and correct management of cases, vaccination, prophylaxis. The latter, that in most cases is performed with isoniazid (300 mg/daily for 12 months) is indicated in the following situations: subjects with > 5 mm tuberculin test; recent contacts with patients with infective tuberculosis; chest X-ray indicative for old fibrotic lesions, HIV infection; subjects with > 10 mm tuberculin test: HIV-negative drug-addicts; clinical conditions at high risk for tuberculosis (e.g. silicosis, hematologic malignancy, iatrogenic immunosuppression).