ABSTRACT. A 7-month-old boy presented with vomiting and failure to thrive associated with proteinuria, methylmalonic aciduria and macrocytosis, but without anaemia. Plasma vitamin B12 levels were normal by a radio-dilution method. He was treated as an inborn error of metabolism with intramuscular cyanocobalamin and a low protein diet. However when treatment was withdrawn he remained well for 11 months before relapsing with vomiting and weight loss. Re-investigation again showed methylmalonic aciduria, but the haemoglobin was low and plasma vitamin B12 levels by a specific method showed them to be reduced. Studies of vitamin B12 absorption showed the picture of selective malabsorption. He was started on regular cyanocobalamin injections.