A method of treating a patient by implanting a system comprising an infusion device for injecting a substance, at least one reservoir comprising at least one compartment which accommodates and preserves the substance to be injected, and an active cooling device adapted to cool the reservoir and thereby keep the substance within said at least one compartment of the reservoir at a temperature below 37° C. The method comprises the steps of: cutting the skin, dissecting free at least one area within the patient's body, placing the infusion device and the at least one reservoir inside the patient's body along with the cooling device, such that the at least one reservoir is in fluid connection with the infusion device to supply to the infusion device the substance to be injected into the patient's body, and closing at least the skin after implantation of at least parts of the system.