The direct 1,9-dimethylmethylene blue (DMB) method for quantifying sulfated glycosaminoglycan (GAG) in urine (Clin Chem 1989; 35: 374-9) has been adapted to a convenient means for sample collection and transport as a test to identify individuals with mucopolysaccharidosis (MPS) storage diseases. Results correlated moderately well (r = 0.85) with those of a commonly used, but more laborious, quantitative method. In studying factors to maximize diferentiation of pathological from normal values, we found that GAG excretion (expressed as milligrams GAG per gram creatinine) fits a logarithmic function with respect to age and varies markedly below age five years. This must be considered in developing normative values and forming diagnoses. Of 112 separate urine specimens obtained from 41 MPS patients representing the major MPS diseases, glycosaminoglycan excretion by all exceeded that for age-matched normal individuals. The convenience of this method allowed us to establish the first normative values for three-week-old infants (n = 435) found to have a mean glycosaminoglycan excretion of 179 (SD 86.3) mg of GAG per gram of creatinine. This method improves the diagnostic capability for those MPS diseases that have been particularly difficult to identify (Sanfilippo's syndrome and Morquio's syndrome), and may also provide a test for other disorders with previously unrecognized abnormal excretion of glycosaminoglycan (e.g., mucolipidosis and acromesomelic dysplasia). Most importantly, this MPS diagnostic test is unique in its suitability for mass screening of newborn infants.