The prevalence of nephrolithiasis is considerably greater in patients with autosomal dominant polycystic kidney disease (ADPKD) than in the general population. We evaluated anatomic and metabolic factors that may be associated with an increased prevalence of nephrolithiasis in patients with ADPKD. We compared anatomic parameters among ADPKD patients with or without nephrolithiasis as diagnosed by ultrasonography, whereas metabolic factors were determined by 24-hour urinary chemical analysis. Patients with ADPKD and nephrolithiasis had more renal cysts (P < 0.05) and a larger predominant renal cyst size (P < 0.005) than patients without nephrolithiasis. Concurrently, individual stone-forming kidneys had a greater cyst number (P < 0.05) and a significantly larger predominant cyst size (P < 0.01) compared with kidneys without stones. Patients with ADPKD and nephrolithiasis had a significantly lower creatinine clearance than those without nephrolithiasis (68.7 +/- 8.6 versus 94.8 +/- 5.4 mL/min, respectively; P < 0.05). Twenty-four-hour urinary analysis showed that patients with ADPKD and nephrolithiasis had significantly lower urinary volumes (P < 0. 05), and levels of urinary phosphate (P < 0.05), magnesium (P < 0. 005), and potassium (P < 0.05). Although not statistically significant, patients with ADPKD with stones tended to have lower levels of urinary citrate, and both groups showed a high percentage (range, 49% to 60%) of patients with hypocitraturia. Our data are consistent with the hypothesis that patients with ADPKD who develop nephrolithiasis do so because of increased intrarenal anatomic obstruction, as well as lower levels of such urinary inhibitors of stones as magnesium and citrate.