Prior to this study, cilostazol, an antithrombotic drug, was thought to exist as a single crystalline phase with a melting point of ∼159°C (Form A). On cooling, melts often form a glass that, when heated, may crystallize as additional crystalline polymorphic forms. Cilostazol, when reheated, subsequently forms polymorphs that melt at ∼136°C (Form B) and 146°C (Form C). Free-energy temperature diagrams estimated from calorimetry data reveal that each pair of the cilostazol polymorphs (A–B, B–C, and A–C) is monotropic. Essentially pure samples of suitable crystalline shape and size permitted single crystal structural analysis of Forms A and C. Theoretical solubility ratios calculated using calorimetry data indicate that at 37°C, Form B should be more than four times more soluble and Form C should be more than two times more soluble than Form A. Forms B and C could not be crystallized from solvents. Metastable forms from super cooled melts analyzed by intrinsic dissolution and Fourier transform-Raman experiments demonstrated that Forms B and C undergo a rapid, solvent-mediated recrystallization to Form A, making dissolution rate measurements difficult. © 2002 Wiley-Liss, Inc. and the American Pharmaceutical Association