IEEE 802.15.4 technology provides one solution for low-rate short range communications. Based on the integrated superframe structure of IEEE 802.15.4, a novel low-delay traffic-adaptive medium access control (LDTA-MAC) protocol for wireless body area networks (WBANs) is proposed in the paper. In LDTA-MAC, the guaranteed time slots (GTSs) are allocated dynamically according to the traffic load. At the same time, the active portion of superframe is kept to be a reasonable duration to decrease the energy consumption of the network devices. Moreover, for the successful GTS requests, the related data packets are transmitted in the current superframe instead of waiting more time to reduce the average packet delay. Simulations are conducted to evaluate the network performance and verify our protocol design. Comparing with IEEE 802.15.4, the results reveal LDTA-MAC accommodates more devices access to the network and reduces the packet delay obviously without the cost of more energy consumption.