The urinary excretion of hypoxanthine, xanthine, and uric acid was measured prior to and following chemotherapy in 11 patients with rapidly growing chemotherapy-sensitive lymphomas who were receiving concomitant allopurinol therapy. Mean maximal total dairy urinary excretions of these purines postchemotherapy were: uric acid, 807 mg/day; hypoxanthine, 343 mg/day; and xanthine, 638 mg/day. The mean maximal postchemotherapy urinary concentrations of uric acid, hypoxanthine, and xanthine were 288, 115, and 179 mg/liter, respectively. Mean total daily urinary excretion of uric acid, hypoxanthine, and xanthine rose 2.2-, 6.6-, and 6.9-fold, respectively, following initiation of antineoplastic therapy. Although standard doses of allopurinol did not prevent a postchemotherapy increase in the excretion of uric acid or hypoxanthine, the urinary concentrations of both compounds remained below their solubility in urine at pH 7 in all 11 patients studied. However, the urinary concentration of xanthine exceeded its solubility in urine at pH 7 in six of the 11 patients. In three of the six patients whose urinary concentration of xanthine concentration exceeded its solubility in urine, transient renal failure developed in association with the increased excretion of xanthine. These studies indicate that, despite the use of conventional doses of allopurinol, the urinary excretion of uric acid may still increase following massive tumor lysis, and urinary excretion of xanthine can increase to concentrations potentially causing xanthine nephropathy.