A selective, voluntary urine screening program has been established to facilitate detection and early treatment of infants with methylmalonic acidurias (MMA), a group of rare, potentially lethal, autosomal recessive disorders of organic acid metabolism. The laboratory methods have been modified for newborn infants so that urine specimens can be collected on filter paper in the diaper and tested by the thin-layer chromatography method. One Hutterite child was previously known to have methylmalonyl-coenzyme A (MMCoA) mutase deficiency (mut0) which is unresponsive to vitamin B12 but is responsive to diet and other therapeutic measures. No undiagnosed existing cases of MMA were identified by the voluntary screening program among 1,165 Hutterite infants and preschool children.