Homocystinuria represents a group of hereditary metabolic disorders characterized by an accumulation of homocysteine in the serum and an increased excretion of homocysteine in the urine. The infantile form is severe: the main clinical findings are neurologic signs, associated with hematological signs and bone alterations. Immediate restoration of plasma amino acids is the primary goal and early diagnosis is crucial not to delay the onset of possible treatment. We report a case of homocystinuria with early onset: an initial symptomatology was undervalued by the pediatrician with a delay in diagnosis. Despite the therapy, the patient developed tetraventricular hydrocephalus requiring ventricular drainage. In conclusion, we want to remember the necessity to perform a complete metabolic workup in a patient with clinical manifestations suggestive for homocystinuria, and the importance of early recognition of the signs and symptoms of hypertensive hydrocephalus, a possible complication of this condition.