This study was performed to evaluate the recommended neuroimaging guidelines using the degree of internal and external dysfunction to avoid unnecessary catheter angiography (CA) for screening isolated oculomotor nerve palsy (IOP). We analyzed the causes and clinical characteristics of 28 consecutive patients with IOP between January 1992 and August 1999. By investigating the association of the degree of internal and external dysfunction with the etiologies of IOP, and by applying guidelines (modified by us) to these patients, we evaluated the usefulness of those guidelines. When we strictly applied the modified guidelines to our patients, CA should have been performed in 13 out of 28 patients (46.4%). These were five patients with normal pupil and partial external dysfunction (PED) due to youth or absence of pathological vascular risk factors (four ischemic; one migrainous). In addition, there were four patients with partial internal dysfunction (PID) and PED (one ischemic; one migrainous and two neoplastic); one with PID and complete external dysfunction (CED) due to young age (ischemic); one with complete internal dysfunction (CID) and PED (aneurysmal) and two with CID and CED (aneurysmal). Ten of 13 patients (76.9%) would have unnecessary CA, but no patients with aneurysms would be missed using these revised guidelines. Modified neuroimaging guidelines, using the degree of internal and external dysfunction, seem to be useful in the management of IOP.