The refereed proceedings of the 6th IEEE International Conference on High Speed Networking and Multimedia Communication, HSNMC 2003, held in Estoril, Portugal in July 2003. The 57 revised full papers presented were carefully reviewed and selected from 105 submissions. The papers are organized in topical sections on integrated differentiated services, multicasting, peer-to- peer networking, quality of service, QoS, network and information management, WDM networks, mobile and wireless networks, video, CDMA, real time issues and protocols for IP networks, multimedia streaming, TCP performance, voice over IP, and traffic models.