Registration of deaths in children under five years of age has improved in Mexico, however, the underreporting has not yet been eliminated.

Background: The underreporting of vital statistics poses a problem for the quality of information. To address underreporting, Mexico implemented the "Intentional Search for Children Deaths" in 2002.

Objective: To analyze trends in the underreporting of deaths in neonates and children under 5 years of age (U5) from 1992 to 2022 at the national level and by state.

Material and methods: A comparative analysis was conducted between the records of the National Institute of Statistics and Geography (INEGI) and the Institute for Health Metrics and Evaluation (IHME) chosen as the gold standard after comparison with five other independent data sources. The analysis considers marginalization to describe the magnitude of underreporting.

Results: From 2001 to 2022, at the national level, the underreporting gap decreased from 38.2% to 9.0% among children U5 and from 30.7% to 5.5% in the neonatal age, with heterogeneous results across states.

Conclusions: The registration of deaths has improved; however, it is crucial to identify states needing further investigation and intervention to reduce the underreporting gap in infant mortality.