Interpretation and modeling of filtercake leakoff experiments are presented. Crosslinked polymer fracturing fluids were used under static conditions. Relationships between ratios of leakoff coefficients and polymer loadings were determined. These can lead to the calculation of any value if one is obtained experimentally. It was determined that the filtercake behavior deviated from the square-root of pressure dependence for an incompressible cake. Experiments, using a constant pressure differential across the cake, indicate relationships between leakoff coefficient and pressure raised to powers approximately 0.25 or less depending on the pressure value, whereas previous work reported a single proportionality for all pressures. Since pressure varies continuously throughout the pumping and closing of a fracture treatment it was imitated experimentally resulting in different pressure differentials at different times. The observed behavior is explained introducing the concept or filtercake resistance. Data interpretation for changing pressures indicates an apparent fluid-loss hysteresis, characterized by the behavior of polymer cakes subjected to changing pressures and by additional cake build-up. A more fundamental interpretation is provided from the theory of viscoelasticity. The hydraulic filtercake resistance, proportional to a rate-normalized pressure difference, leads to a stress-sensitive skin-effect, exactly analogous to similar situations in pressure transient analysis.