Food purchasing and preparation practices were modified in two boarding high schools to increase the polyunsaturated-to-saturated fat ratio (P/S) of the diet of students by changing food products rather than attempting to change eating behaviors. During years when fat-modified products were served, the P/S of males increased by 75 percent, versus a decrease of 6 percent during control years. For females, P/S increased by 53 percent during intervention years, versus an increase of 6 percent during control years.