In vitro degradation of poly(L-lactic acid) (PLLA) fibers was studied by incubating the fibers in phosphate buffered saline (pH 7.4) at 37degreesC for 35-45 weeks. Three kinds of PLLA fibers with different diameters and different initial viscosity-average molecular weights were tested. The viscosity-average molecular weight, crystallinity, morphology, and tensile properties of the PLLA fibers were monitored along the degradation period. The results showed that the viscosity-average molecular weight of the PLLA fibers dropped gradually during the in vitro degradation period, and it decreased by 40-60% at the end of the experimental period. The crystallinity of the PLLA fibers measured by differential scanning calorimetry was noted to increase slightly. The tensile moduli, ultimate strengths, and elongations of the fibers did not exhibit significant changes in 35 weeks of degradation but PLLA fibers with a smaller diameter of 113 mum showed significant decreases in their ultimate strengths with respect to the initial value after 35-45 weeks of degradation. Microcracks appeared on part of the fiber surfaces, which might ultimately lead to the decrease of the mechanical strength of the PLLA fibers. The spotty defects reported in this study apparently did not affect the mechanical properties of the fibers. The results suggested that the tensile properties of the PLLA fibers were mechanically stable in 35 weeks of in vitro degradation, although their molecular weights decreased remarkably and obvious spotty defects appeared on their surfaces.