This is an historical survey of the reasons for the rapid increase in life expectancy and decline in infant mortality in Costa Rica in this century, with regression analysis of determinants of infant mortality. Costa Rica, although a small, relatively poor Central American country, has as of 1985 a life expectancy of 74 years and infant mortality rate of 19/1000. Some of the general social features contributing to its success are racial and cultural homogeneity, constitutional renunciation of an army, a social-democratic welfare-oriented government since the 1940's and a universal education. In recent decades institutional reorganization included formation of a Central Sanitary Office in the Ministry of Health, a Central Assistance Office, and a Social Security System providing medical and hospital care. In the 1970s all hospitals were placed under Social Security. Now 98.9% of hospitalizations are covered, and 7% of the GDP goes for health care. The epidemiological transition began gradually with sanitation and economic growth in the early 20th century and accelerated with the advent of antibiotics, vaccines, and DDT in the 1940s and 1950s. After a stagnant period in the 1960s due to volcano eruptions, the 1970s saw vast improvements in education, communication, and health infrastructure resulting in control of malaria, tuberculosis, helminthiases, tetanus, measles, diarrheal and respiratory infectious deaths. In the 1970s, primary health care, in the form of quarterly home visits to disadvantaged rural and urban areas, equalized health indices across the country. Immunization reached 95% an sanitation 96% of homes. Secondary health care (outpatient) was extended under Social Security. In this era, fertility fell by half, primarily due to birth spacing and limiting of higher order births. Regression analysis shows that primary health care accounted for 41%, secondary medical care for 32%, socioeconomic progress for 22%, and decline in fertility for 5% of the fall in infant mortality.