Background: In the study of the factors that affect infant mortality the interaction of variables has been poorly explored. Objective: Exemplify a method to assess interaction between two variables associated to infant mortality (maternal and paternal educational level). Materials and Methods: Based on data published in the Demography Yearbook (2007), infant mortality rate (IMR) was calculated for 9 combinations of maternal and paternal years of schooling (0-9, 10-12 y 13 + years) and the IMR ratio between combinations. Interaction was evaluated by comparing the sum of the attributable risk of discordant combinations (0-9 years of schooling mother and 13+ years of schooling father, and vice versa) versus the combination (13+years of schooling mother and father). Results: IMR of the combination (0-9 years of schooling mother and father) was 2.7 fold the combination (13+ years of schooling mother and father), being also greater than the sum of the attributable risk of the discordant combinations. Conclusion: The existence of a synergic interaction in additive scale between maternal and paternal education level was demonstrated. Identifying the presence of interactions leads to a focalized implementation of sanitary policies, raising their effectiveness.