An isolated third nerve palsy presenting as the primary manifestation of a lymphoma is rare, with only few cases having been described. The present study reports an unusual case of a healthy 67-year old male diagnosed with isolated right oculomotor nerve palsy (ONP), who was found to have an underlying B cell lymphoma. The patient's medical records were accessed upon consent. A thorough physical examination, including stroke and infections work-ups were performed. A chest computerized tomography (CT), brain magnetic resonance imaging and positron emission tomography (PET) scans and a mediastinal tissue biopsy, were performed as part of systematic diagnostic evaluations. The current report suggests that a PET fluorodeoxyglucose study or a CT scan of the chest, abdomen and pelvis (with contrast) may help in the early diagnosis of a cancer responsible for ONP, particularly if brain vessel imaging does not show a posterior cerebral artery aneurysm as a cause for the defect.