Besides hydrolysis, the physical properties of poly(L-lactic acid), PLLA, may change, at a shorter timescale, at physiological temperature, because of structural relaxation. This process may be relevant as it may contribute for the time-dependent modifications of the mechanical performance of PLLA-based implants, or in other properties such as specific volume or permeability. In this work, microhardness was measured at room temperature after ageing poly(L-lactic acid) samples at 37 degrees C during different periods, ranging from 15 min to 15 days. For the PLLA analyzed, the initial Vickers microhardness was around 140 MPa and an increase higher than 55% was observed in a time span of 15 days. The relaxation of enthalpy at 37 degrees C was also monitored by differential scanning calorimetry, and both results were successfully described by a simple model that considers a nonexponential evolution of the quantities sensitive to physical ageing. The time-scales of the structural relaxation probed by microhardness or by enthalpy were different, and the results suggested that the evolution of the mechanical property was slower than the later. (c) 2006 Wiley Periodicals, Inc.