A method and a system for monitoring a spatial area in a personnel interlock are provided. Image data are acquired which represent a plurality of depth images of the spatial area which have been acquired simultaneously and from different positions in the area of the personnel interlock, in each case by a respective 3D image sensor arranged at the corresponding position. For each of the depth images, an associated characteristic depth image value is calculated. A total image value is obtained by combining the characteristic depth image values of the various depth images in accordance with a predetermined monotonic combination rule. An estimation result for the probability that more than one person is present in the spatial area is then determined.