Although BCG vaccination is recommended for tuberculin negative adult immigrants, there has hitherto been no evidence of its efficacy in this group. This epidemiological study compares the incidence of tuberculosis in a cohort of South Asian adult immigrants vaccinated on entry to the UK with its incidence in cohorts of immigrants of the same age and origin estimated from the five-yearly national surveys. Results suggest that BCG vaccine may have reduced the incidence of tuberculosis by up to 87.6% (95% confidence interval 55.2% to 98.5%) and support the current recommendation to vaccinate this high-risk group.