In a visual recognition task for random patterns, event-related potentials (ERPs) were recorded concurrently with the behavioral performance. A prove stimulus, the feature of which was physically different from a sample stimuli to be memorized, elicited a posterior negative wave in the 220-280-ms latency range. Stimulus complexity and longer retention interval decreased not only the accuracy of behavioral performance but also the amplitude of the negative wave. Furthermore, when the subject failed to recognize the difference, the negative wave was not observed. The posterior negative wave might, therefore, reflect traces of visual short-term memory.