Wireless Body Area Networks (WBAN) combined with various body sensors and personal electronic devices are emerging as a new platform for medical applications. This paper proposes a real-time monitoring system with complex window query support for enabling advanced monitoring on streaming data that is produced by the body sensors and transmitted through the WBAN. The main techniques developed are the caching for multi-source data stream support, complex range expression processing with interval skip lists, and historic data processing for sliding window condition monitoring support. The condition monitoring query language and efficient processing mechanisms to support the complex monitoring are presented. Experimental results show that the proposed approach provides a wider spectrum of monitoring functions as well as enhancing the scalability and efficiency of the monitoring.