An accumulator is employed in an implantable infusion device to provide compliance in the flow path of the device. The accumulator may act to increase the pumping accuracy and repeatability while simultaneously reducing the energy requirements of the device. In one example, the accumulator is arranged at the outlet of a fluid delivery pump of the infusion device. The accumulator includes a cover and a diaphragm biased away from the cover and configured to deflect toward the cover under pressure generated by the therapeutic agent in a flow path of the infusion device. The cover of the accumulator is configured to withstand the pressure generated by the therapeutic agent in the flow path without deforming.