Recent studies suggested that air pollution might be related to low birth weight. We tested this hypothesis on data from the British 1946 birth cohort. We found a strong association between birth weight and an air pollution index based on coal consumption. Babies born in the most polluted areas were on average 87 grams lighter than those born in the areas with the cleanest air. Adjustment for a number of sociodemographic factors did not change these estimates. While confounding by unmeasured factors cannot be ruled out, these historical data support the hypothesis that birth weight could be affected by air pollution.