We present a 9-month-old exclusively breast-fed baby of a strict vegetarian mother who had excluded all animal proteins from her diet. The patient's symptoms included dystrophy, weakness, muscular atrophy, loss of tendon reflexes, psychomotor regression and haematological abnormalities. Biochemical investigations revealed severe methylmalonic aciduria and homocystinuria in the patient, slight methylmalonic aciduria in the mother and low concentrations of serum vitamin B12 in both patient and mother.