Objective: To assess the effect of mining pollution on birthweight. Design: A retrospective before-and-after study with an untreated comparison group. Setting: La Oroya, a mining town in the Peruvian Andes, considered the most contaminated town in the Andean region. Population: All pregnant women who delivered in the social security healthcare system in years 2005, 2006, 2008 and 2009. A total of 214 983 births records were used, 957 from La Oroya and 214 026 from the rest of the country. Methods: A difference-in-difference estimation is used to assess the effect of mining pollution on birthweight before and after two business policy changes: a partial environmental improvement and a subsequent closure of smelter operations in La Oroya. Birthweight was compared with a group not affected by the environmental changes in La Oroya. Main outcome measures: Birthweight in grams. Results: A steep reduction in mining pollution due to the closure of smelter operations in La Oroya showed an increased birthweight of 71.6 g after controlling for socio-economic and medical characteristics. None of the environmental improvements prior to the closure had a statistically significant effect on birthweight. Conclusions: Mining pollution in La Oroya had a negative impact on birthweight. Partial environmental improvements were not enough to improve birthweight. Only after the closure of all mining and smelter operations in La Oroya was a significant gain in birthweight shown. Tweetable abstract: The closure of the most contaminated mine site in the Andean region increased birthweight by 72 g.