Information filtering systems constitute a critical component in modern information seeking applications. As the number of users grows and the information available becomes even bigger it is crucial to employ scalable and efficient representation and filtering techniques. In this paper we propose an innovative XML filtering system that utilizes clustering of user profiles in order to reduce the filtering space and achieves sub-linear filtering time. The proposed system employs a unique sequence representation for user profiles and XML documents based on the depth-first traversal of the XML tree and an appropriate distance metric in order to compare and cluster the user profiles and filter the incoming XML documents. Experimental results depict that the proposed system outperforms the previous approaches in XML filtering and achieves sub-linear filtering time.