An improved ambulatory infusion device for dispensing a fluid into a patient includes a flexible bladder enclosure including a sheath of flexible material having a first open end a closed second end. The first open end of the sheath is affixed to a rigid closure, and a bladder support is secured in fixed position relative to the closure. An elastic bladder is mounted on the bladder support and positioned inside the sheath for holding fluid to be dispensed. The closed second end of the sheath is free to conform to the body of the patient so that the device is comfortable when worn. In other aspects, a flow restrictor and a pressure regulator are provided.