Poly(L-lactide-co-epsilon-caprolactone) [P(LLA-CL)], which is used in biodegradable biomedical materials such as drug-delivery systems, surgical sutures, orthopedics, and scaffolds for tissue engineering, has been reported to crystallize upon storage in a dry state even at room temperature; this results in rapid changes in the mechanical properties. hi biomedical applications, P(LLA-CL) is used in the presence of water. This study investigated the effects of water on the crystallization of P(LLA-CL) at 37 degrees C in phosphate buffered solution, which was anticipated to alter its mechanical properties and hydrolytic degradation behavior. Surprisingly, the crystallinity of P(LLA-CL) in the presence of water rapidly increased in 6-12 h and then slowly increased up to 120 h. The period of time for the initial rapid crystallization increase in the presence of water was much shorter than that in the absence of water. The obtained information would be useful for the selection, preparation, and use of P(LLA-CL) in various biomedical applications. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 112: 715-720, 2009