Here, we present a case of tonic pupil associated with occult breast cancer as a paraneoplastic neuro-ophthalmology syndrome. A 45-year-old woman developed progressive photophobia and blurred vision due to unilateral Adie's tonic pupil. Magnetic resonance image of her brain and neurological examination (including deep tendon reflexes) were normal at first visit. Follow-up examinations performed by ophthalmologist every 6 month without any change in her condition. After 2 years, patient discovered a mass in her breast which identified to be malignant after diagnostic procedures. Despite surgical and medical treatment for cancer, no change in the ocular condition was happened.