Autoimmunity against collapsin response-mediator protein-5 (anti-CRMP-5) has been associated with ocular inflammation in paraneoplastic syndrome. We present a 59-year-old Caucasian man with optic neuritis and vitreous cells in both eyes (OU), at different stages. Despite the fact that the patient did not have any systemic disease, we suspected a paraneoplastic syndrome and requested CRMP-5-IgG and a mediastinoscopy. After performing the tests, a small cell lung carcinoma was diagnosed. Autoantibody CRMP-5-IgG positivity and optic neuritis combined with vitreous inflammation was defined as a paraneoplastic entity, avoiding vitreous biopsy and allowing us to suspect malignancy before systemic symptoms appeared.