Background: The prevalence of childhood "overweight" and "at risk for overweight" has become a major public health concern. School food environments can affect key nutritional risk factors, especially in high schools where foods of poor nutrient value are pervasive in a la carte and vending programs. This study examines a la carte and vending programs in Maine public high schools at baseline and following implementation of low-fat (defined as items less than or equal to 30% of total calories from fat), low-sugar (defined as items less than or equal to 35% of sugar by weight), and portion-controlled (LFLS) guidelines. Methods: Four high schools implemented LFLS guidelines and 3 made no changes to a la carte and vending programs for a period of 1 school year. Results: Findings revealed no significant change in food and beverage offerings in control schools. Whereas, in intervention schools, the proportion of items meeting the LFLS nutrient criteria increased from 32.8% to 81.8% in a la carte items, increased from 22.5% to 84.0% in snack vending, and increased from 48.0% to 98.9% in beverage vending from baseline to follow-up. However, these increases were mitigated when LFLS portion size criteria were applied. Conclusions: These findings demonstrate the successful implementation of LFLS guidelines similar to recommendations recently proposed by numerous organizations. School food service personnel were identified as key stakeholders in the successful implementation of the LFLS guidelines. Furthermore, these findings emphasize an important role the food and beverage industry will have in providing foods and beverages that meet proposed nutrient and portion guidelines. (Contains 2 tables.)