Failure to thrive is a frequent cause of consultation in pediatric services. The main objective in these patients is the early detection of an organic cause, if present. We report a case of low-grade astrocytoma of the optic pathway in a 2-month-old child whose main symptoms at diagnosis were failure to thrive and anorexia. Unfortunately, despite therapeutic efforts, the tumor showed local and metastatic progression refractory to chemotherapy. The patient died 3 months after diagnosis. We conclude that diencephalic tumors must be considered in the differential diagnosis of failure to thrive during the first year of life, especially when, after initial investigations, a cause is not found.