The hydrolytic degradation of poly(lactic acid) (PLA) devices has previously been reported as size dependent for devices such as plates, microspheres, and films between 2 and 0.3 mm in thickness or diameter. In this study, the effect of fiber diameter on the degradation characteristics of PLA fiber of two diameters, 32 mu m (PLA32) and 118 mu m (PLA118), aged at 40, 60, and 80 degrees C with 100% relative humidity, was investigated. Additionally, both PLA32 and PLA118 were aged at 40 and 60 degrees C under nitrogen purge. The degradation of the fibers was evaluated based on changes in the total weight, crystallinity, and molecular weight of the samples. Both diameters exhibited similar total weight loss, crystallinity, and molecular weight loss profiles under each set of degradation conditions. Two models for the change in molecular weight were compared: a first order model and an autocatalytic model. For the obtained data, both models provided a reasonable fit of the molecular weight data. Based on the rate constants obtained for each model, the activation energy for PLA degradation was calculated (26.3 kcal mol(-1) for the first order model and 22.4 kcal mol(-1) for the autocatalytic model). The activation energies obtained were close to other values reported in the literature for PLA hydrolysis. POLYM. ENG. SCI., 55:1652-1660, 2015. (c) 2014 Society of Plastics Engineers