To determine if there are differences in urinary glycosaminoglycan (GAG) concentrations, 43 stone-forming patients and 37 healthy control subjects of both sexes were studied. Urinary concentrations of calcium, magnesium, creatinine, uric acid and GAGs were determined. GAGs were measured by the Di Ferrante precipitation procedure followed by the Bitter and Muir reaction. Urinary GAG concentration and daily output were significantly lower in stone-forming patients. The present study clearly demonstrates the decreased urinary GAG concentration and excretion in stone-forming patients and suggests an interaction between GAGs and urate that could modify the inhibitory potency of GAGs.