Abstract: An arrangement for the detection of fraudulent use of a telephone subscriber's instrument in a mobile telephone system includes an input preprocessor (110), a neural network engine (111) coupled to the preprocessor, and an output postprocessor (112) coupled to the neural network engine. The preprocessor determines for each subscriber a first long term calling profile, a second short term calling profile, and a subscriber profile pattern comprising the difference between the first and second profiles. Each calling profile and subscriber profile pattern comprises a set of values for a respective set of call attributes. The neural network engine comprises a self organizing map trained to effect pattern recognition of the subscriber profile patterns and a multilayer perceptron adapted to determine for each recognized pattern a value indicative of the probability of a fraud being associated with that pattern.