Congenital disorders of glycosylation include a group of diseases, each of them caused by different protein (mostly enzyme) impairment due to a specific gene defect. The many subtypes are classified according to clinical features, enzymology and molecular genetic analyses. Problems in diagnostics arise from the great diversity in clinical presentation, usually age-related, and different severities of individual types of these, by far underdiagnosed, diseases. Also the biochemical findings tend to vary, even within a single type. No one screening test, common for all types, is available so far. Several methods of choice may be used in the first approach; other procedures must follow for detailed typing of the defect. Possible drawbacks and pitfalls in the diagnostics from the viewpoint of our 3-year studies and practical screening experience are presented.