PURPOSE: Cranberry supplements are commonly used as a natural deterrent to urinary tract infection. However, one small study (n=5) which showed an increase in urinary oxalate levels following cranberry supplementation has led to its use with caution among patients susceptible to nephrolithiasis. Furthermore, most commonly available cranberry tablet preparations contain vitamin C, which has been independently shown to increase urinary oxalate excretion. The aim of this study is to investigate the influence of cranberry supplementation on urinary oxalate excretion. METHODS: Fifteen participants were randomised to receive cranberry tablets alone or cranberry tablets containing vitamin C. Tablets were taken at the manufacturers recommended dosage for a period of 14 days. Participants provided a 24 h urine collection at trial entry and day 14. Urinary variables were compared to assess for changes in oxalate levels. RESULTS: The median age was 27 years (21-43). There was no difference in the 24 h urine volume pre or post commencement of cranberry tablets (1.7 vs 2 L, p=0.07). An increase in median urinary oxalate excretion was observed in participants taking both cranberry-only tablets (0.10 mmol/day) and tablets containing vitamin C (1.15 mmol/day). CONCLUSION: Dietary supplementation with cranberry increases urinary oxalate excretion and should be avoided in patients at risk of urolithiasis.