Wireless sensor networks have today emerged as a feasible infrastructure for healthcare applications. This paper addresses the non-trivial performance problems in contentionbased wireless networks. We present a method for admission control in contention-based networks, implemented as a component of a performance management system. The test results show that admission control can improve the predictability and level of performance in wireless sensor networks. The system can be used as a tool for dimensioning and configuration as well as for real-time admission control. The often unpredictable dynamics in contention-based access networks means that continuous performance control is needed to maintain a desired quality of service.