Water-filled, drop impact tests on full-scale, blow-molding polyester bottles showed that impact properties change significantly after 30 days of aging at 50 degrees C. To quantify this deleterious effect of aging, we did small-scale laboratory experiments using the essential work of fracture method (EWFM). We measured the essential work of fracture of thin specimens cut from the sides of blow-molded bottles and aged under various conditions. Similar to the impact results, significant drops in essential work occurred after relatively short amounts of aging. Additional experiments confirmed that the decreases in essential work were due to physical aging and not to chemical degradation. A comparison of the essential work results to the full-scale, bottle-drop impact tests showed some inconsistencies that need to be resolved before EWFM can be recommended as a quantitative method for studying physical aging of blow-molding polyester resins.