Questions have been raised about the effectiveness of Bacille Calmette-Guérin (BCG) vaccination against tuberculosis (TB) in adults. We therefore analysed the immune response after BCG vaccination in primary-vaccinated and revaccinated young adults. 31 tuberculin skin test (TST) negative healthy students were BCG-vaccinated; 15 were primary-vaccinated and 16 revaccinated. Tuberculin-induced lymphocyte transformation (LT) and cytokine production of peripheral blood mononuclear cells were studied before BCG vaccination, as well as after 2 months and 1 y. In the primary-vaccinated as well as the revaccinated group the LT response increased after 2 months and remained significantly higher than baseline values after 1 y. In both groups the interferon-gamma (IFN-gamma) levels increased significantly after 2 months and the increase was maintained after 1 y. LT increased more in the revaccinated group than in the primary-vaccinated group, while the increase in IFN-gamma response did not differ between the 2 groups. Both primary vaccination and revaccination of TST negative young adults caused a significant increase in the T-helper 1-type immune response, suggesting a protective effect against TB. The present in vitro results thus support the policy in several low-endemic countries of primary vaccination as well as revaccination of young adults at risk of TB exposure.