Anatomical and physiological investigations in monkeys indicate that olfaction is subserved by several cortical regions. But the areas implicated in the human olfactory system have not been definitively identified by functional criteria. Behavioural evidence has suggested that laterally specialized mechanisms for odour processing may exist, but the neuroanatomical substrate remains unknown. We used positron emission tomography to study the cortical representation of human olfactory processing by comparing cerebral blood flow changes evoked during olfactory stimulation with those of a control task. We report here significant cerebral blood flow increases at the junction of the inferior frontal and temporal lobes bilaterally, corresponding to the piriform cortex, and unilaterally, in the right orbitofrontal cortex. The results complement and extend previous data implicating these regions in olfactory processing, and indicate that a functional asymmetry exists in the human brain favouring the right orbitofrontal area in olfaction.