In this protocol, we will clinically evaluate patients with methymalonic acidemia and cobalamin metabolic defects. Routine inpatient admissions will last 4-5 days and involve urine collection, blood drawing, ophthalmological examination, radiological procedures, MRI/MRS, skin biopsies in some, and developmental testing. In a subset of patients who have or will receive renal, hepato- or hepato-renal transplants or have an unusual variant or clinical course and have MMA, a lumbar puncture to examine CSF metabolites will be performed. In this small group of patients, CSF metabolite monitoring may be used to adjust therapy.