Stochastic bending fluctuations of biomembranes are of high physiological importance, e.g., for preventing adhesion between red blood cells or for the establishment of cell substrate contacts in cell adhesion. Unfortunately, especially in nucleated cells they are notoriously difficult to quantify. To meet this challenge we developed a new technique based on Fluorescence Correlation Spectroscopy (FCS). We use comparatively high labeling of the biomembrane (∼10000 fluorophores in the focal spot) to suppress all contributions of lateral diffusion to fluctuations of the fluorescence intensity. Under these conditions all intensity variations arise from axial motion of the membrane. Best results are achieved when the focus is placed slightly above or below the membrane. The resolution, as estimated for ideal membranes is 20 nm spatial and 10 µs temporal.