We report on the isothermal crystallization behavior of thin (film thickness d < 500 nm) and ultrathin (d < 100 nm) films of poly(ethylene oxide) (PEO), as well as pyrene end-labeled PEO, on native silicon studied by in situ hot stage atomic force microscopy (AFM). Individual lamellae were imaged during crystallization and melting. Using AFM, we have directly measured lamellar growth rates, lamellar thicknesses, and melting ranges as a function of film thickness (ca. 15- >500 nm), crystallization temperature (40-62 degreesC), and molar mass (11-100 kg/mol). On the basis of the Hoffman-Weeks extrapolation, the Gibbs-Thomson equation, and the Hoffman-Lauritzen theory, we show that the crystallization of PEO in thin and ultrathin films can be described with the same laws as the bulk crystallization. In addition, we find that the equilibrium melting points and surface free energies of the fold surfaces agree quantitatively with literature data for bulk crystallization and hence are not altered due to confinement in ultrathin films. However, there is a monotonic decrease of lamellar growth rates with decreasing film thickness for films thinner than ca. 250 nm. The growth rates decrease to below 1% of their bulk value in the thinnest films; this is attributed to an increase in glass transition temperature of up to 30 degreesC for the confined PEO and the concomitant reduction of molecular mobility.