Wireless body area networks (WBANs) have become one of the key components of mobile health (mHealth) which provides 24/7 health monitoring service and greatly improves the quality and efficiency of healthcare. However, users’ concern about the security and privacy of their health information has become one of the major obstacles that impede the wide adoption of WBANs. Anonymous and unlinkable authentication is critical to protect the security and privacy of sensitive physiological information in transit from the client to the application provider. We first show that the anonymous authentication scheme of Wang and Zhang based on bilinear pairing is prone to client impersonation attack. Then, we propose an enhanced anonymous authentication scheme to remedy the flaw in Wang and Zhang’s scheme. We give the security analysis to demonstrate that the enhanced scheme achieves the desired security features and withstands various known attacks.