Design and sample. This randomized effectiveness trial on children 12-30 mo of age was conducted in 4 States of Central and Eastern Mexico. The randomized effectiveness trial was conducted in 12 milk distribution clusters (MDC) comprising periurban areas serving around 5000 beneficiaries with one or more milk distribution centers. These clusters were randomly selected from a universe of all MDC (n=542) in 4 States in central and eastern Mexico. The twelve clusters were randomly assigned to receive either fortified (n=7) or non-fortified (n=5) milk using a random numbers list. The clusters assigned to the non-fortified milk started receiving fortified milk at the end of the trial. The milk distributed to the participants in the trial was all in the dry form.