Abstract: An arrangement for detecting leakage in a hydraulic system of a working vehicle, which hydraulic system comprises a tank (1) for hydraulic oil, hydraulic working means such as hydraulic cylinders (6), which can be acted upon by the hydraulic oil and conduits (7a, 7b) which connect the hydraulic working means (6) to the tank (1), which arrangement for detecting leakage comprises a supervision system (5) and a level sensor (2) which is arranged in the tank (1) to send signals to the supervision system (5) in order to reflect the current hydraulic oil volume in the tank. The supervision system is adapted to using these signals for calculating a volume change rate, and the level sensor (2) is adapted to operating continuously, or at regular brief intervals of time, when the vehicle is in a dynamic state, i.e. when the vehicle is in motion or any of the vehicle's working means (6) are being used.