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A wireless sensor network is a network of compact micro-sensors equipped with wireless communication capability. In a wireless sensor network, saving energy is a critical issue. Furthermore, a sensor node is expected to face many difficulties in signaling and computing. As a MAC scheme for a wireless sensor network, we thus propose an energy-aware version of pure ALOHA scheme, where rather than sacrificing the simplicity of pure ALOHA, we take a straightforward approach in saving energy by trading off throughput performance. First, we add a step of deciding between stop and continuation prior to each delivery attempt for a MAC PDU. Secondly, we find an optimal stopping rule for such a decision in consideration of the losses reflecting energy consumption as well as throughput degradation. In particular, we note that the results of delivery attempts are hardly predictable in the environment that sensor nodes contend for the error-prone wireless resource. Thus, presuming that only partial information about such results is available to sensor nodes, we explicitly draw an optimal stopping rule. Finally, numerical examples are given to demonstrate the expected losses incurred by optimal stopping rules with full and partial information.