Mounting evidence suggests that early-life conditions have an enduring effect on an individual’s mortality risks as an adult. The contribution of improvements in early-life conditions to the overall decline in adult mortality, however, remains a debated issue. We provide an estimate of the contribution of improvements in early-life conditions to mortality decline after age 30 in Dutch cohorts born between 1812 and 1921. We used two proxies for early-life conditions: median height and early-childhood mortality. We estimate that improvements in early-life conditions contributed more than five years or about a third to the rise in women’s life expectancy at age 30. Improvements in early-life conditions contributed almost three years or more than a quarter to the rise in men’s life expectancy at age 30. Height appears to be the more important of the two proxies for early-life conditions.