Tuberculosis (TB) notification is a statutory requirement in Hong Kong, where contact investigations are performed by the Tuberculosis and Chest Service. 1) To evaluate the risk of active TB in close contacts within 5 years, and 2) to identify risk factors associated with early and late development of active TB disease. The characteristics of consecutive TB cases notified from 18 January to 17 April 2000 were collected together with those of their contacts. Contacts were prospectively followed up through the territory-wide TB notification registry for 5 years for the development of disease. A total of 1537 index cases and 4661 close contacts were analysed. Screening found 31 (0.67%) active TB cases within a 3-month period, and another 58 (1.24%) cases presented subsequently. Index cases with cough or pulmonary cavities and diabetic contacts were independent risk factors of early cases (all P<0.05). Adjusted at risk index characteristics for late TB development included positive sputum smear (2.79, 95%CI 1.31-5.95) and family history of TB (4.26, 95%CI 2.01-9.03). Contact risk factors included diabetes mellitus (3.44, 95%CI 1.04-11.33) and institutionalisation (3.61, 95%CI 1.70-7.65). Considerable TB risk remains after initial contact screening. A number of possible risk factors were identified.