Olfactory hypersensitivity is defined as a partial or total decrease, loss or abnormality of olfactory function. The olfactory nerve is the nerve fiber that runs from the olfactory epithelium through the sieve plate to the olfactory bulb. The ability to smell is a characteristic of the olfactory cells in the nasal mucosa. Injury to the nasal mucosa, olfactory bulb, olfactory filament or central nervous system connections may affect the sense of smell. Clinical manifestations are hyposmia, olfactory loss, olfactory deficit, olfactory inversion, phantom smell and increased sensitivity to olfactory stimuli. The pathogenesis of olfactory hypersensitivity and the molecular biology of olfaction are unknown. Among them, nasal mucosa, olfactory bulb, and olfactory filament neuropathy cause decreased or lost olfactory function; whereas damage to the central nervous system connections is usually not accompanied by any detectable olfactory loss. Olfactory deficits occur due to embryonic abnormalities in the genesis of the olfactory nerve. Occasionally, temporal lobe lesions are accompanied by temporary or paroxysmal phantom smells. Olfactory loss is often combined with taste impairment, depending on volatile substances in food and beverages. Olfactory impairment due to influenza is temporary.