Human olfactory discrimination after unilateral frontal or temporal lobectomy. Olfactory discrimination and detection was studied in 106 patients with unilateral cerebral excision in the right or left temporal lobe, right or left frontal lobe, left parietal lobe, or right frontal and temporal lobes, and in 20 normal control subjects. Detection thresholds for n-butyl alcohol, measured separately in each nostril, did not differ across subject groups or across nostrils, thus excluding any primary sensory loss. The discrimination task involved monorhinal presentation of repairs of unfamiliar odorants, which the subjects judged as same or different in quality. The results showed a significant deficit in discrimination confined to the nostril ipsilateral to the lesion in patients with temporal lobe removals. Patients with frontal lobe excisions were also impaired and, for patients with right frontal lesions including the orbital cortex, the impairment was found in both nostrils. Patients with left parietal lesions did not demonstrate any significant deficits. Normal subjects showed consistently better performance in the right than in the left nostril. The results are interpreted as reflecting the importance of the orbitofrontal cortex in olfactory discrimination. Temporal lobe lesions may disrupt the input to the orbitofrontal cortex, thereby producing poorer performance. The nostril difference in the normal subjects, together with the birhinal impairment in patients with right orbitofrontal damage, suggest a relative advantage of the right orbital region in olfactory processing. 