Developmental regression is the presenting symptom of most infants with cobalamin (Vitamin B12) deficiency. We present a report of three infants with cobalamin deficiency in which the infants also developed a movement disorder. In each case the mother was a vegetarian and the infant was exclusively breast-fed. In two of the infants, a striking movement disorder consisting of a combination of tremor and myoclonus particularly involving face, tongue, and pharynx appeared 48 h after the initiation of treatment with intramuscular cobalamin. This was associated with marked changes in plasma amino acid levels. Paradoxically, the onset of the movement disorder coincided with overall neurological improvement. The third infant had a persistent focal tremor, which appeared before the commencement of treatment. The movements slowly abated during a 3-6 week period. The presence of a movement disorder in cobalamin deficiency has received less attention than other features, but in a mild form is probably common. It may offer an early clue to the diagnosis before the onset of profound neurological deterioration. The cause of the severe movement disorder that can appear after treatment is not known.