Humans are readily able to distinguish expected and unexpected sensory events. Whether a single mechanism underlies this ability is unknown. The most common type of expected sensory events are those generated as a consequence of self-generated actions. Using H2 15O PET, we studied brain responses to such predictable sensory events (tones) and to similar unpredictable events and especially how the processing of predictable sensory events is modified by the context of a causative self-generated action. Increases in activity when the tones were unpredictable were seen in the inferior and superior temporal lobe bilaterally, the right parahippocampal gyrus and right parietal cortex. Self-generated actions produced activity in a number of motor and premotor areas, including dorsolateral prefrontal cortex. We observed an interaction between the predictability of stimuli and self-generated actions in several areas, including the medial posterior cingulate cortex, left insula, dorsomedial thalamus, superior colliculus and right inferior temporal cortex. This modulation of activity associated with stimulus predictability in the context of self-generated actions implies that these areas may be involved in self-monitoring processes. Detection of expected stimuli and the detection of the sensory consequences of self-generated actions appear to be functionally distinct processes, and are carried out in different cortical areas. These observations support theoretical approaches to cognition that postulate the existence of a self-monitoring system.