A randomized, controlled trial measured the short-term impact of the Peruvian school breakfast programme on the diet, school attendance, and cognition of fourth- and fifth-graders. Ten schools on the rural outskirts of the Andean city of Huaraz were randomly assigned to a control or a treatment group. The programme significantly increased (p < .01) dietary in-takes of energy by 15.2%, protein by 16.1%, and iron by 60%, and improved rates of attendance. Analysis of covariance also showed improved performance on a vocabulary test among heavier children, as indicated by a positive and significant weight × treatment interaction (parameter = 0.37; F = 4.97; < .05). It is plausible that among stunted children, being relatively overweight (by the weight-for-height index) poses a cognitive risk that can be partially reversed by a school breakfast.