Xanthinuria typ I is a very rare disorder of purine metabolism caused by xanthine oxidoreductase (XDH, EC 1.1.1.204.) deficiency. We report the first case of this deficiency in Poland discovered in a 78-year-old woman treated for rheumatoid arthritis. Hypouricemia was a fortuitous discovery revealed during routine tests. She had major xanthinuria and a radiotranslucid lithiasis in left kidney. Hereditary xanthinuria is characterized by hypouricemia, hypouricosuria and increased concentration of oxipurines in plasma and urine. Development of xanthine lithiasis is directly related to the low solubility of xanthine in urine and is the main complication of the disease, occurring in approximately 40% of patients. There is no effective treatment and the only useful measure is to prevent xanthine lithiasis by maintaining high fluid intake.