We conducted a case-control study to evaluate the effectiveness of BCG vaccination in preventing childhood tuberculosis (TB) in Cali, Colombia. We ascertained 178 cases aged 0 to 14 years from the respiratory clinics with cough or fever for at least three weeks and a positive chest X-ray for TB, as well as 320 controls who were from the same households but had no symptoms and negative X-rays. Using matched set multiple logistic regression analysis, we found the age- and sex-adjusted relative risk (RR) of TB among vaccinees compared with non-vaccinees to be 0.84 with 95% confidence limits (CL) from 0.43 to 1.62. There was, however, a significantly lowered relative risk of TB with increasing time since vaccination (RR = 0.83 per year since time of vaccination with 95% CL from 0.74 to 0.94.)