Horner's syndrome consists of an injury to the sympathetic pupillary pathway, Described with the triad: ptosis, miosis, and anhidrosis. Recognizing the signs of this syndrome, their levels of injury and causes are of paramount importance to the clinician. Case: A 38-year-old female patient presented with ptosis of the upper eyelid, right eye, 2 months of evolution, growth of axillary ganglion 4 years ago, also cough of 6-month duration, and 6 kg weight loss in 3 months. Exploration: mild ptosis, greater anisocoria in the dark, and right miosis. Systemic exploration: right breast tumor of 6 cm, adhered to deep planes. The patient was referred to an oncological center. This case is important because there is a widespread of prevention programs; however, there are still people who continue to hide their diseases, and we must not forget that the eyeball is a key point to detect systemic diseases.