Abstract In order to investigate the relationship between the structural change and the endothermic heat at the main transition in dipalmitoylphosphatidylcholine (DPPC)-water systems with three different water concentrations (60, 25 and 15 wt%), we have performed simultaneous differential scanning calorimetry (DSC) and time-resolved X-ray diffraction. The changes of wide-angle X-ray diffraction pattern for the hydrocarbon chain packing were compared quantitatively with the endothermic heat at the main transition. In contrast to the fully hydrated sample with water concentration 60 wrt%, a broad and asymmetric curve was observed in the DSC thermogram in the samples with the water content, 15 and 25 wt%. On the other hand, the wide-angle X-ray diffraction patterns recorded simultaneously suggest that the gel and the liquid-crystalline phases coexist in the transition region. The fraction of the liquid-crystalline phase is estimated from the wide-angle X-ray diffraction profiles in the coexistence region. This fraction agrees with the normalized integrated DSC curves. Therefore, the endothermic heat at the main transition of DPPC correlates largely with the melting of the hydrocarbon chains.