A door system for a motor vehicle includes a door, an environmental sensor for detecting an obstacle in the vicinity of the motor vehicle, a door sensor for outputting an output signal as a function of the initiation of the opening of the door, and a control unit for detecting an imminent collision between the door and the obstacle as a function of an output signal of the environmental sensor and for triggering a warning device such that a warning may be output by the warning device in response to the detection of an imminent collision between the door and the obstacle. During the triggering of the warning device, the control unit distinguishes between at least two selectable warning stages, a first warning stage and a second warning stage, and the selection of a warning stage by the control unit is a function of the output signal of the door sensor.