An environmental program directed at the food service departments of two boarding high schools has been tested in a concurrently controlled longitudinal investigation in which the intervention was applied to each school in alternate years. It has been demonstrated that changes in food purchasing and preparation practices can markedly decrease sodium and modify the fat composition of foods, and that such practices result in significant changes in the nutrient intake of students. Even without an educational component for students, who maintained their usual dietary practices, the changes by food service workers led to 15-20% less sodium intake, 20% less saturated fat intake, and an increase in the P/S ratio from 0.46 to 0.84 among students. The change in sodium intake over a school year resulted in lower blood pressure among students receiving the intervention. Adjusting for sex and baseline blood pressure, the estimated effect of the intervention on systolic pressure was -1.7 mmHg (95% C.I. -0.6, -2.9; p = 0.003); for diastolic pressure, it was -1.5 mmHg (95% C.I. -0.6, -2.5; p = 0.002). Such modifications by school food service workers are well accepted and produce very palatable foods. The widespread dissemination of such practices could favorably affect cardiovascular risk factors of students everywhere.