Laminar flow over an enclosed rotating disk was studied to reduce the inconsistency between previous theoretical and experimental results. Unlike the case of a disk in an infinite fluid medium, the friction-moment coefficient of the enclosed disk is proportional to Re−1 in the laminar range and Re−1/4 in the turbulent range. The latter is accurate for the range of disk diameter to gap ratios between 200 and 50. The former correlation, instead of being an approximation according to simple shear as has been suggested, is shown to be true even when recirculation exists. The deviation from Re−1 relation in the laminar range is shown to be due to the inertia effect of recirculation. The significance of inertia effects in addition to that due to centrifugal force has been pointed out and corrections have been presented. Radial outflow has been shown to be more effective than radial inflow for turbine-disk cooling. It has been shown that rotation of the shroud provides an added sealing effect for a centrifugal machine.