The service lifetime and performance for a prosthetic and orthotic thermoplastic, Subortholen(TM), was measured using correlations between artificial and natural weathering degradation data of (1) the accumulated carbonyl compound concentration at the polymer surface, (2) changes in thickness of the surface reaction zone, and (3) changes in hardness and tensile properties. An empirical, mathematical relationship between artificial and natural weathering exposures is presented. Results show that the hardness degradation is a useful parameter to estimate the different stages in clinical performance of Subortholen. Corroborating hardness, tensile properties, and weathering exposure time, the service lifetime for Subortholen was determined to be 4-6 years. This finding indicates that the correlation between artificial and natural weathering degradation may provide an effective way to calculate the most appropriate time to replace a worn prosthetic or orthotic device before the device passes into a dangerous stage.