Poly(L-lactide) [i.e., poly(L-lactic acid) (PLLA)] was hydrolyzed in the melt in high-temperature and high-pressure water at the temperature range of 180-350 degreesC for a period of 30 min, and formation, racemization, and decomposition of lactic acids and molecular weight change of PLLA were investigated. The highest maximum yield Of L-lactic acid, ca. 90%, was attained at 250 degreesC in the hydrolysis periods of 10-20 min. Too-high hydrolysis temperatures such as 350 degreesC induce the dramatic racemization and decomposition of formed lactic acids, resulting in decreased maximum yield Of L-lactic acid. The hydrolysis of PLLA proceeds homogeneously and randomly via a bulk erosion mechanism. The molecular weight of PLLA decreased exponentially without formation of low-molecular-weight specific peaks originating from crystalline residues. The activation energy for the hydrolysis (DeltaE(h)) of PLLA in the melt (180-250 degreesC) was 12.2 kcal.mol(-1), which is lower than 20.0 kcal.mol(-1) for PLLA and 19.9 kcal.mol(-1) for poly(DL-lactide) [i.e., poly(DL-lactic aicd)] as a solid in the temperature range below the glass-transition temperature (21-45 degreesC). This study reveals that hydrolysis of PLLA in the melt is an effective and simple method to obtain L-lactic acid and to prepare PLLA having different molecular weights without containing the specific low-molecular-weight chains, because of the removal of the effect caused by crystalline residues.