Both hyperuricaemia and hyperuricosuria have been considered to be common in renal calcium stone disease. Abnormalities in serum or urinary urate was not more common in 450 investigated patients than in 90 healthy controls, but when present it was often associated with a more complicated course of the disease, particularly in combination with renal acidification defects. Hyperuricosuria was not explained by a renal tubular dysfunction of urate transport. A decreased urate clearance in proximal renal acidification dysfunction was associated with a high tubular reabsorption of filtered urate. Sodium urate or uric acid crystals did not adsorb urinary macro-molecular inhibitors of crystal growth other than in calcium oxalate solution, whereby a substantial loss of the inhibitory activity was detected. A high intake of dietary animal protein had a unfavourable effect, as several urinary constituents potentially important for calcium stone formation were aggravated. The urinary excretion of calcium and urate increased, whereas urine pH and urinary citrate decreased. It was concluded that the protein content of the diet may be an important factor contributing to the increase in the renal calcium stone disease in the industrialized world.