Vitamin B12 deficiency is a very rare disease of infants and young children in Europe. Authors report a case of a 9.5-month-old infant who was exclusively breast-fed by his vegan mother and developed serious vitamin B12 deficiency in form of neurological regression, repetitive vomiting, drowsiness, dysphagia, obstipation, and tremor. A few days after intramuscular vitamin substitution his abnormal signs improved dramatically, hematological restitution was reached in six weeks. Authors describe the hematological and neurological signs, the diagnostic and differential-diagnostic pitfalls, therapy, prognosis, and prevention of this condition. Beside reviewing the literature they emphasize the importance of early recognition and intervention and the need of an appropriate doctor-parent cooperation in this disease.