There is evidence suggesting that glycosaminoglycans (GAG) are potent inhibitors of growth and aggregation of calcium oxalate crystals in vitro. This finding raises the possibility that the urinary GAG could play an inhibitory role in the urolithiasis. To investigate this hypothesis, a study on the urinary excretion of GAG in normal and stone forming adults and children was undertaken. Different methods were compared, and the best results were obtained when the GAG were measured by densitometry after agarose gel electrophoresis. Although the GAG concentration was increased in the morning urine compared to the 24-hour urine samples, and in males compared to females, the GAG/creatinine ratio was independent of period of urine collection and of sex. So, it was advantageous to express the amounts of urinary GAG as mg/g of creatinine. Children excreted more GAG than adults, with a higher proportion of chondroitin sulfate. We have shown that the stone forming subjects, both adults and children, excreted lower levels of urinary GAG as compared to normal subjects, independently of the metabolic disorder. The proportions between chondroitin sulfate and heparan sulfate and the structures of these GAG were unaltered in the stone formers. These results indicate that there is a definite difference in terms of levels of GAG between normal and stone forming urines, and suggest a correlation between the urinary GAG concentration and urolithiasis.