Carbohydrate-deficient glycoprotein syndrome is a recently identified recessively inherited, multisystemic disease with severe nervous system involvement. It is characterized biochemically by carbohydrate-deficient serum glycoproteins, and can be diagnosed by analysis of abnormal isoforms of serum transferrin. Using stored, neonatally collected filter paper blood spots from such patients, it was shown that neonatal diagnosis was possible by immune-isoelectric focusing of transferrin eluted from up to 14-year-old samples. Freshly collected blood on filter paper was readily analyzed quantitatively for carbohydrate-deficient isotransferrins by a rapid microchromatographic assay, revealing highly elevated values in all patients. The presently described methods thus provide a means for early diagnosis of the carbohydrate-deficient glycoprotein syndrome in microliter volumes of capillary blood. Sampling on filter paper offers an important simplification in sample collection, storage and transport, and may make population studies possible.