OBJECTIVE To investigate the potential influence of cranberry juice on urinary biochemical and physicochemical risk factors associated with the formation of calcium oxalate kidney stones, as this product might affect the chemical composition of urine. METHODS Urinary variables were assessed in a randomized cross-over trial in 20 South African men (students) with no previous history of kidney stones. The first group of 10 subjects drank 500 mL of cranberry juice diluted with 1500 mL tap water for 2 weeks, while the second group drank 2000 mL of tap water for the same period. This was followed by a 2-week 'washout' period before the two groups crossed over. During the experimental phase subjects kept a 3-day food diary to assess their dietary and fluid intakes; 24-h urine samples were collected at baseline and on day 14 of the trial periods, and analysed using modern laboratory techniques. Urine analysis data were used to calculate the relative urinary supersaturations of calcium oxalate, uric acid and calcium phosphate. Data were assessed statistically by analysis of variance. RESULTS The ingestion of cranberry juice significantly and uniquely altered three key urinary risk factors. Oxalate and phosphate excretion decreased while citrate excretion increased. In addition, there was a decrease in the relative supersaturation of calcium oxalate, which tended to be significantly lower than that induced by water alone. CONCLUSIONS Cranberry juice has antilithogenic properties and, as such, deserves consideration as a conservative therapeutic protocol in managing calcium oxalate urolithiasis.