The effect of thermal treatment on the crystalline and supermolecular structure of polylactide was investigated. The samples were isothermally crystallized by two methods, the crystallization temperature being reached (1) via cooling from the melt and (2) via heating from the glassy, amorphous state. The samples were crystallized over a broad temperature range of 70-130degreesC. The glassy state was produced by the quenching of the polymer melt to ambient temperature. The morphology and structural features of the samples were examined with X-ray techniques (wide-angle and small-angle X-ray scattering), small-angle light scattering, polarizing light microscopy, and differential scanning calorimetry. The results showed similar structures on the lamellar level for samples prepared by the two methods under similar time and temperature conditions. Crystal perfection increased with the crystallization temperature. This was accompanied by an evolution of the spherulitic structure: the spherulites were larger in samples crystallized via cooling from the melt than those in samples crystallized via heating from the glassy, amorphous state. The thermal properties were influenced markedly by the initial crystallinity of the polymer, but differentiation of the supermolecular structures between samples crystallized according to the two protocols did not affect their thermal behavior. The structure of each sample was correlated with its viscoelastic properties. The viscoelastic response was sensitive to the crystallinity level but depended less on the sizes of the spherulites.