In this paper, a Cognitive Radio Based Medium Access Control (CR‐MAC) protocol for Wireless Sensor Body Area Networks (WSBAN) that utilizes cognitive radio transmission is proposed. In this proposal, the sensor nodes are classified into nodes of life‐critical health information and nodes of non‐critical health information. The CR‐MAC protocol prioritizes the critical packets access to the transmission medium by transmitting them with higher power while transmitting lower priority packets using lower transmission power. At the receiver, a higher priority packet experiences collision only when there are more than one critical packet transmission at the same transmission slot while non critical packets experience collision when there are more than one transmission at the same transmission slot. This protocol is evaluated analytically and through simulation. The obtained results demonstrate a differentiated service system which prioritizes critical traffic access to the transmission medium and increases the critical traffic throughput. Copyright © 2010 John Wiley & Sons, Ltd.