Uric acid is the end product of purine degradation in humans. It is a weak acid with a pK of 5.35 in urine. In acidic urine the undissociated form of uric acid predominates and is poorly soluble, leading to crystalluria and stone formation. Uric acid stones account for 5% to 10% of all stones in the United States. Acidic urine, hyperuricosuria, and low urine volume are the risk factors for uric acid stone formation. Uric acid stones may be idiopathic or secondary to a systemic disease such as gout. Treatment includes alkalinization of the urine, high fluid intake, and reduction of uric acid excretion if it is abnormal.