Feasibility of microcalorimetry to evaluate the physical stability of amorphous drugs was studied. Amorphous forms of nifedipine and phenobarbital were prepared by melting and subsequent cooling in a differential scanning calorimetry (DSC) sample pan, and their heats of crystallization were monitored by isothermal microcalorimetry. The time required for 10% of the amorphous drug to crystallize (t(90)), a direct measure of the crystallization rate, could be obtained from a single microcalorimetric trace of the amorphous nifedipine or phenobarbital. The t(90) values were also determined by conventional storage studies in which the heat of crystallization was determined by DSC. The t(90) values obtained by microcalorimetry were consistent with those obtained by DSC, within experimental error, indicating that microcalorimetry is a useful method for evaluating the physical stability of amorphous drugs.