The etiological diagnosis of the third nerve palsy frequently imposes a real problem. Therefore, we have attempted to review the different causes by studying 109 personal cases and those described in the literature. Among our 109 cases, 36% were related to an aneurys,, 16% to a tumour, 12% to vascular diseases. Twenty-seven percent of our cases could not be clearly explained and were regarded as "neuritis". In total, sudden and isolated palsy, aneurysm is the first etiology to be considered. However the high incidence of palsies of unknown origin is striking and has been observed by other authors. The various etiological diagnosis of the third nerve palsy are also discussed, as well as the paraclinical investigations to be performed. In addition to the general and neurological examinations, analysis of the CSF, dosage of the PBI and of the glycemia, serologic tests for syphilis, endrophonium (Tensilon) and neostigmine tests, plain skull radiographies as well as frontal and posterior planigraphies of the cranial sinuses, must be kept in mind. Angiography and air encephlography will exclude intracerebral organic lesions. When ptosis and progressive ophtalmplegia are associated, and myasthenia or hyperthyroidism have been excluded, myopathy must be considered and a muscle biopsy may be performed. Finally when symptoms and signs of multineuritis are present and when expansive or vascular lesions of the brain stem has been excluded, examination of the CSF would sometimes help in the diagnosis of encephalitis.