Ultrahigh molecular weight polyethylene (UHMWPE) is injection molded under high pressure and studied by ultra-small-angle X-ray scattering (USAXS) during melting in a time-resolved synchrotron radiation experiment. Results concerning melting and recrystallization of crystalline lamellae are compared to data obtained by differential scanning calorimetry (DSC). USAXS analysis reveals a coupled process of melting and crystallization which is not accompanied by external heat flow. Nine isotropic samples differing in molecular weight and molding pressure are heated at a rate of 5 degreesC/min. 2D USAXS images integrate over temperature intervals between 3 and 7 degreesC. The materials are considered two-phase semicrystalline polymers. Scattering curves obtained by azimuthal averaging are transformed to interface distribution functions (IDF) which are perfectly Fitted by a nanostructural model comprising an ensemble of thick, uncorrelated layers (50 nm thickness) and stacks of short-range correlated crystalline lamellae (20 nm). Crystalline layers are identified from their narrower layer thickness distribution and their melting behavior, After the scattering effect of amorphous layers is eliminated, a composite crystallite thickness distribution is obtained. Its variation is studied as a function of temperature, molecular mass and molding pressure. In DSC thermograms samples prepared at high pressure exhibit a single strong melting peak, whereas the other samples show an additional melting peak at lower temperature. This might lead to the conclusion that the high-pressure samples predominantly contain extended chain crystals. USAXS shows that high-pressure materials contain considerable amounts of imperfect thin crystal lamellae that melt at lower temperature, while thick lamellae are formed. With low-pressure samples, this coupled process of nanostructure transformation during annealing is found to be negligible.