Methods and apparatus, including computer program products, implementing and using techniques for pattern detection in input data containing several transactions, each transaction having at least one item. Filter conditions for interesting patterns are received, and a first set of filter conditions applicable in connection with generation of candidate patterns is determined. An evaluated candidate pattern is selected as a parent candidate pattern, and evaluation information about the parent candidate pattern is maintained. Child candidate patterns are generated by extending the parent candidate pattern and taking into account the first set of filter conditions. The child candidate patterns are evaluated with respect to the input data together in sets of similar candidate patterns and based on the evaluation information about the parent candidate pattern. At least one child candidate pattern successfully passing the evaluation step is recursively used as a parent candidate pattern.