Energy efficiency is an essential design issue for wireless sensor networks (WSNs). Single-hop dynamic clustering proposed by LEACH (Low-Energy Adaptive Clustering Hierarchy) is a simple and energy-efficient approach in WSNs. Due to the particular requirements of RF power of wireless sensors, LEACH may only be applicable to small-scale WSNs. In this paper, we propose an adaptive clustering protocol for medium-scale WSNs (ACPM) that could be applied to larger networks than LEACH. ACPM adopts the adaptive back-off scheme for cluster head selection to achieve its goal. Simulation results show that ACPM performs better than LEACH, especially in medium-scale WSNs.