The invention relates to a method for measuring deflection of a substantially elongated hollow component (B) of a wind energy system from a normal position, extended such that it can be used to determine the deflection of the hollow component (B) of a of a wind energy system from a normal position in a simple and accurate manner, and to reliably monitor the same. To this end, the invention proposes that at least one distance sensor (8.1) for measuring distance in a non-contacting, directed manner is disposed at a first position, taking distance measurements from a target point in the direction of a second position within the hollow component (B), and that a measurement target (9.1) is further disposed in the interior of the hollow component (B) at the second position, said target comprising a target surface extending in a direction (x) transverse to a reference plane (R) perpendicular to the measurement direction (10) of the distance sensor (8.1) in a measurement area, and that distance measurements to the target surface are taken continuously or at intervals by the distance sensor (8.1), wherein the results of the distance measurements are evaluated in a processing unit, and an associated deflection of the hollow component (B) from the normal position is determined from said measurements, each with regard to the directional component of the direction (x) in which the target surface extends transverse to the reference plane (R).