<span name="style_bold">Background:</span> Infant mortality has always been a very good health indicator because it is easy to calculate and to interpret. It also reflects very well the living conditions and health progress of a country. This work aims to evaluate the evolution of infant mortality rate over a period of ninety years (1920-2009). <span name="style_bold">Methods:</span> A descriptive epidemiological study on the evolution of infant mortality over a ninety-year period. In order to calculate infant mortality rates, we obtained from INEC the number of deaths and live births for each year. The average mortality rate for five-year periods was also calculated. Infant mortality was divided in neonatal and post neonatal mortality. It was also possible to obtain heall-cause mortality rates for each year, and it was used to determine the percentage of infant deaths with respect to total deaths. Results: In 1920, infant mortality rate was 248.3 per thousand live births and in 2009 it was 8.8; a 96.4% decrease. In 1920, the neonatal mortality rate was 67.6 per thousand live births and in 2009 it was 6.44; a 90.5% decrease. The postneonatal mortality rate went from 180.7 in 1920 to 2.4 in 2009; a 98.6% decrease. Until 1974, the postneonatal/neonatal mortality ratio always favored the former, but this changed and for 1995-99 the figure for neonatal mortality was twice the one for postneonatal mortality, and for 2005-09 it was almost threefold. Before 1965-1969, infant mortality accounted for more than 30% of the all-cause mortality, and in certain years it was over 40%. Since 1970, the figure of infant mortality as a percentage of all-cause death dropped, until it reached 3.9% during 2005-2009. A descriptive epidemiological study on the evolution of infant mortality over a ninety-year period. In order to calculate infant mortality rates, we obtained from INEC the number of deaths and live births for each year. The average mortality rate for five-year periods was also calculated. Infant mortality was divided in neonatal and post neonatal mortality. It was also possible to obtain heall-cause mortality rates for each year, and it was used to determine the percentage of infant deaths with respect to total deaths. Results: In 1920, infant mortality rate was 248.3 per thousand live births and in 2009 it was 8.8; a 96.4% decrease. In 1920, the neonatal mortality rate was 67.6 per thousand live births and in 2009 it was 6.44; a 90.5% decrease. The postneonatal mortality rate went from 180.7 in 1920 to 2.4 in 2009; a 98.6% decrease. Until 1974, the postneonatal/neonatal mortality ratio always favored the former, but this changed and for 1995-99 the figure for neonatal mortality was twice the one for postneonatal mortality, and for 2005-09 it was almost threefold. Before 1965-1969, infant mortality accounted for more than 30% of the all-cause mortality, and in certain years it was over 40%. Since 1970, the figure of infant mortality as a percentage of all-cause death dropped, until it reached 3.9% during 2005-2009.

Public health; descriptive epidemiology; infant mortality