Medical, in particular dentistry, turbine handpiece having a gripping sleeve and a head piece which is disposed on a front end of the gripping sleeve and has a tool receptacle opening, wherein the gripping sleeve has a fluid line for supplying a fluid for driving a turbine wheel, wherein the turbine wheel and a holding means for rotatably mounting a treatment tool are disposed in the interior of the head piece, wherein the holding means is arranged along a rotation axis for the treatment tool, and wherein furthermore disposed in the interior of the head piece along the rotation axis is a releasing element which is configured to interact with the holding means, wherein the releasing element when interacting with the holding means releases the mounting of the treatment tool, wherein the head piece furthermore has a cover which is lies opposite the tool receptacle opening and which is adjustable in the direction of the rotation axis between a resting position, in which the cover is spaced apart from the releasing element, and an activating position, in which the cover presses on the releasing element in order to release the mounting of the treatment tool, and wherein a sealing element is disposed in the head piece, characterized in that the sealing element is effective between the cover and a stationary bearing element which annularly surrounds the holding means, wherein the sealing element is fastened either to the cover or to the bearing element, and by way of a preload presses against a sealing surface of the bearing element or of the cover, and wherein the sealing element is embodied as a valve and is configured to be released from the sealing surface in the activated state of the handpiece. wherein the gripping sleeve has a fluid line for supplying a fluid for driving a turbine wheel, wherein the turbine wheel and a holding means for rotatably mounting a treatment tool are disposed in the interior of the head piece, wherein the holding means is arranged along a rotation axis for the treatment tool, and wherein furthermore disposed in the interior of the head piece along the rotation axis is a releasing element which is configured to interact with the holding means, wherein the releasing element when interacting with the holding means releases the mounting of the treatment tool, wherein the head piece furthermore has a cover which is lies opposite the tool receptacle opening and which is adjustable in the direction of the rotation axis between a resting position, in which the cover is spaced apart from the releasing element, and an activating position, in which the cover presses on the releasing element in order to release the mounting of the treatment tool, and wherein a sealing element is disposed in the head piece, characterized in that the sealing element is effective between the cover and a stationary bearing element which annularly surrounds the holding means, wherein the sealing element is fastened either to the cover or to the bearing element, and by way of a preload presses against a sealing surface of the bearing element or of the cover, and wherein the sealing element is embodied as a valve and is configured to be released from the sealing surface in the activated state of the handpiece.