We have prospectively studied 37 adult patients (15 males, 22 females; age 31 +/- 10.6 years) with previously undiagnosed isolated hematuria in which hypercalciuria or hyperuricosuria was found. Eighteen of them had had episodes of gross hematuria. Isolated hypercalciuria (4.4 to 10.4, X 5.6 +/- 1.9 mg/kg/24 hr) was found in nine patients (Group I), isolated hyperuricosuria (784 to 1500, X 1088 +/- 228 mg/24 hr) in 11 (Group II), and both hypercalciuria (4 to 8, X 4.9 +/- 1 mg/kg/24 hr) and hyperuricosuria (752 to 1476, X 1042 +/- 181 mg/24 hr) in 17 patients (Group III). Thiazide treatment for patients with hypercalciuria and allopurinol for those with hyperuricosuria were administered; calciuria and uricosuria became normal by the first month of therapy in every case. In 22 (59.4%) cases (Responder patients) hematuria resolved completely as soon as calciuria and uricosuria became normal. In the remaining 15 cases (Nonresponder patients) hematuria persisted despite the normal calcium and uric acid excretions. Several disorders that explained hematuria were diagnosed later in most of Nonresponder patients. Responder patients persisted without hematuria on the follow-up; only in three patients a transient relapse of hematuria was seen associated with a sudden increase of calciuria and uricosuria because of treatment withdrawal. There were no differences in age, male/female ratio nor in the basal values of calciuria and uricosuria between Responder and Nonresponder patients. A familial history of urolithiasis was found more frequently in Responder patients (64%) than in Nonresponders (20%) (P less than 0.05). We conclude that hypercalciuria and hyperuricosuria are definable and potentially reversible causes of hematuria in adult patients.