OBJECTIVE In patients with a percutaneous nephrostomy tube (PCN) inserted for symptomatic stone disease, antegrade pyelography is an accepted modality to assess the collecting system and residual stone status prior to PCN removal. Recently, unenhanced multidetector CT (UMDCT) has shown its superiority for the assessment of urinary-tract stones. Comparison of UMDCT with antegrade pyelography has never been done; hence, our aim was to compare the two methods for the assessment of urinary stones in patients with a PCN. METHODS Between July 2004 and July 2005, we prospectively imaged 49 consecutive patients with known urinary-tract stone disease who had PCN (27 men and 22 women; average age 57 +/- 20 years; range 4-88 years). All patients underwent UMDCT and antegrade pyelography within 24 hours. Both examinations were prospectively and blindly evaluated by two attending radiologists for the presence, location, and size of urinary-tract stones. RESULTS According to the findings of both imaging modalities, 18 patients were stone free, and 31 patients had urinary stones. In 20 of the latter 31 patients (64.5%), the urinary stones were diagnosed only by UMDCT. Antegrade pyelography missed renal as well as ureteral stones, with a significant mean size (5.1 x 6.2 mm, and 6 x 5.3 mm, respectively). Antegrade pyelography missed radiolucent (8/20) as well as radiopaque (12/20) stones. In 11 of the 31 patients (35.5%), urinary stones were diagnosed by both UMDCT and antegrade pyelography. The average size of these renal stones was 6 x 11 mm, and the mean ureteral stone size was 11 x 13 mm. In 64% (7/11), the stones were radiolucent and in 36% (4/11) radiopaque. There was no patient in whom urinary stones were diagnosed by antegrade pyelography but missed by UMDCT. CONCLUSIONS Unenhanced multidetector CT is more accurate than antegrade pyelography via a PCN for the assessment of urinary-tract stones, with the advantage of reducing the risks of contrast injection side effects.