In vitro degradation of poly(l-lactic acid) fibers was investigated for a period of 16 weeks in Ringer solution at 37 degrees C. Two sets of fibers, with similar initial mechanical properties, molar mass, and crystallinity content, but markedly different in diameter (72 and 120 mu m) were studied. Viscometric molar mass decreased during the immersion time at a faster rate for the thinner fibers compared to the thicker ones. As a consequence, the fiber mechanical properties changed; the elastic modulus was only slightly affected by the molar mass decrease whereas ultimate mechanical properties (stress and strain at break) showed a strong decrease. A quantitative correlation between tensile strength and viscometric-average molar mass was attempted. A possible explanation of the faster degradation rate of the thinner fibers was proposed on the basis of the higher surface/volume ratio and water uptake. Dynamic mechanical properties were also measured as a function of immersion time.