Using population-level data from the Mexican Family Life Survey (MxFLS), we examine the impact on child health of a large-scale conditional cash transfer program, Oportunidades. This innovative antipoverty program puts additional resources in the hands of women and their families and encourages parents to invest in human capital of their children. Program income accounts for about 25% of total resources in beneficiary households. The causal impact of the program on child health is isolated by exploiting insights from the biology of child growth in combination with the timing of the roll-out of Oportunidades and the panel dimension of MxFLS. Height for age among children exposed during the first 3 years of life is contrasted with similar children who were not exposed.