A male infant with hyperpipecolic acidemia is described. To our knowledge this is only the second report of this disorder. As with the previous case, our patient's course was characterized by persistent hepatomegaly, severe mental retardation, progressive loss of developmental milestones and diminished visual acuity associated with nystagmus, abnormal discs and retinal changes. Death occurred at 2 years of age, following a progressive loss of neurological function. Pipecolic acid was repeatedly present in the serum at a concentrattion of 4-5 mg %. Trace amounts of this compound were also detected in the urine. In addition, an adaption of the method of Piez et al. (1956) for the direct quantitation of pipecolic acid in serum was evaluated and found to be very useful for the biochemical diagnosis of this disorder.