This study assesses the efficiency of the galactosemia add-on test in neonatal screening performed on regular Guthrie card blood spots. Based on estimated average incidence of galactosemia (1:19,984 newborns) in São Paulo State, Brazil, the study develops a cost-benefit analysis model, using a B/C ratio and a 9.25% annual interest rate in order to decapitalize the results. Sensitivity analysis is also performed, varying (as a function of the interest or discount rate) from 0 and 20% and according to the 95% confidence interval (1:7,494-1:59,953 newborns). The results show that the savings obtained by improved health of galactosemic patients detected early by add-on neonatal screening is superior to the costs (B/C=1.33), characterizing galactosemia add-on testing in neonatal screening as an efficient policy. The lower the prevailing interest rate in the economy, the more efficient the neonatal screening policy.