To enlarge the coverage of tetanus immunization program in preventing neonatal tetanus, there has been a change of the target group from pregnant women to child-bearing age women disregarding their pregnancy status. This change calls for administration of a booster dose to enhance the long-lasting immunity among the child-bearing age women in the community. To determine the proper time to give the booster dose, tetanus antitoxin levels of 21-39 years old women who had received primary immunization at 2-7 years before were measured by passive haemagglutination assay. The women with antitoxin levels of less than 0.01 HAU/ml were assumed to need a booster injection. Regression analysis clearly showed the demand of booster dose was increasing linearly from 40% up to 60% as measured at 2-7 years of post-vaccination period. Geometric mean titres were maintained in between 0.02 - 0.03 HAU/ml up to 5 years of post-vaccination period, then decreased, to 0.015 HAU/ml and 0.0058 HAU/ml in 1 and 2 years later, respectively. Based on the results, and considering some other advantages, it was suggested to administer the booster dose of tetanus toxoid at an earliest opportunity, say at 1 or 2 years after administration of primary immunization.