Acetal is known for its fatigue resistance, however, it is also susceptible to degradation at high temperatures. Commercial resins therefore contain complex stabilizer systems that, although present in relatively small amounts, provide thermal stability both for processing and subsequent in-service aging. The present study was undertaken to determine how sensitive the fatigue resistance of an acetal copolymer was to changes in stabilizer concentration. Blends containing a series of increasing stabilizer levels were investigated and compared to the commercial material. Fatigue resistance was assessed using accelerated fatigue crack propagation rate measurements for injection molded plaques. Results demonstrate that the fatigue resistance is dependent upon the amount of stabilizer and in fact suggest that a critical level of stabilizer is needed to provide adequate fatigue resistance in the molded part. It is also shown that room temperature aging further aggravates the poor fatigue resistance of samples with low stabilizer levels. Moreover, the results from this study demonstrate that the varying concentrations of heat stabilizer affect even short term mechanical properties such as tensile modulus, strength, and elongation to fail. In addition, fracture toughness tests clearly distinguish the various materials. The short term mechanical tests provide added support that a critical concentration of stabilizer exists.