Optical resolution of a new antidiabetic agent, (±)-5-[4-(1-methylcyclohexylmethoxy) benzyl]-2, 4-thiazolidinedione (1, ciglitazone) with (-)-and (+)-1-phenylethylamine (PEA) in ethyl acetate resulted in almost complete asymmetric transformation to give the salts, (-)-1·(-)-PEA and (+)-1·(+)-PEA, respectively, in up to quantitative yields. Optical purities of (-)-and (+)-1 obtained from the salts were determined by nuclear magnetic resonance and their absolute configurations were confirmed chemically. The optical isomers showed essentially the same antidiabetic and hypolipidemic activities.