Aside from its recognition and warning functions, olfaction serves many purposes in the CNS and remains one of the most important means of communication with the environment. In addition to olfactory tract input, the olfactory bulb also receives and provides input to other brain centers that modify neuronal activity. Research in the field of immunology as well as in various brain illnesses is beginning to indicate the increasing relevance of smell in pathophysiology. Much of this is based on the many intricate interactions that exist between the immune system and the nervous system, and evidence exists that there may be something unique about the olfactory system that is inextricably related to immunological function. In addition, accumulating evidence confirms the existence of olfactory dysfunction in brain disease, much of which appears at early stages including multiple sclerosis, Alzheimer's Disease, Parkinson's Disease, schizophrenia and depression. Such observations may further suggest that under certain circumstances, olfactory abnormalities may be associated with autoimmune conditions. Since the organization of the olfactory system is so sensitive, impairment may be noted at an early stage. This may become important in the prediction of certain brain illnesses. While preliminary evidence may suggest a role for olfaction in the management and alleviation of various disorders, investigation of its clinical relevance remains limited.