An arrangement to measure deflection of a blade of a wind turbine is provided. A transmitter is arranged close to the tip end of the blade, while a receiver is arranged close to the root end of the blade. The transmitter and receiver are prepared for a wireless transfer of a monitoring signal, which is sent from the transmitter to the receiver. A monitoring system is arranged close to the root end of the blade. The monitoring system is adapted to generate the monitoring signal. The monitoring system is connected with the transmitter by a cable-bound communication line, thus the monitoring signal is transferred from the monitoring system to the transmitter. The monitoring system is connected with the receiver, thus the monitoring signal is transferred from the receiver to the monitoring system. The monitoring system is adapted to determine the deflection of the blade based on the transferred monitoring signal.