The effect of glycosaminoglycans on calcium oxalate crystal formation in the supersaturated solution was studied by examining the size and shape of calcium oxalate crystals generated under an optical microscope. It was found that heparan sulfate and heparin were more effective growth inhibitors than chondroitin sulfate and hyaluronic acid at concentrations within their respective urinary range. With increasing calcium and/or glycosaminoglycans concentration in the solution, the degree of growth inhibition caused by glycosaminoglycans was enhanced. Calcium oxalate crystal shapes generated with various glycosaminoglycans varied with glycosaminoglycan species. One of the causes of those differences in the shape and degree of growth inhibition might be the structural differences between them, that is, the number of sulfate residue and O- or N-form they contain. Calcium oxalate crystal shapes in the presence of heparin or heparan sulfate at higher concentrations were similar to those of calcium oxalate monohydrate crystals in the urinary sediments of hyperoxaluric patients. These facts might suggest the possibility that heparin and/or heparan sulfate were present in the crystal forming region.