The greater energy content of larger food portions could be contributing to the increasing prevalence of overweight. Prevention guidelines recommend "sensible" portion sizes but do not define them. The US Department of Agriculture (USDA) defines standard serving sizes for dietary guidance, and the Food and Drug Administration (FDA) defines standard servings for food labels. To use these standards in counseling, nutritionists must know the sizes of portions available in the marketplace. We determined marketplace portion sizes, identified changes in these sizes with time, and compared current marketplace portions with federal standards. Most marketplace portions exceed standard serving sizes by at least a factor of 2 and sometimes 8-fold. Portions have increased over time; those offered by fast-food chains, for example, often are 2 to 5 times larger than the original size. The discrepancy between marketplace portions and standard servings suggests the need for greater emphasis on the relationship of portion size to energy intake as a factor in weight maintenance.