Abstract: The present invention relates to an arrangement for monitor¬ing the movement of a piston rod (5) in a pressure medium operated actuator (2). Then, a movement of a machine part controlled by the pressure medium operated actuator (2) is monitored in a space of time and place in order to produce terminal location data (Px) on each movement of this machine part. The arrangement comprising at least one 3-axis sensor unit (1) determining a position in relation to a gravitational vector and storage means (3) for receiving and storing the terminal location data produced by this sensor unit. From each two successive pieces of terminal location data (Px) produced by the sensor unit, a movement distance (Lx) per formed by the piston rod (5) in the actuator is calculated by processing means (4).