To assess prospectively the effect of section width in multi-detector row computed tomographic (CT) evaluation of patients with acute flank pain who are suspected of having or known to have urinary stone disease.This study was approved by the ethics committee of the authors' university, and written informed consent was obtained from all patients. One hundred forty-seven patients (age range, 11-101 years; mean, 51.4 years +/- 18.7 [standard deviation]) suspected of having urinary stone disease underwent unenhanced multi-detector row CT. CT was performed with four detector rows, a section thickness of 1.0 mm, an effective tube current-time product of 100 mAs, and a tube voltage of 120 kVp (CT dose index, 11.4 mGy). From these data, three sets of transverse images were reconstructed with section widths of 1.5, 3.0, and 5.0 mm and approximately 50\% of overlap each. Scans were evaluated in varying random orders by two radiologists for the number, size, and location of uroliths and nephroliths and for the presence of phleboliths, renal cysts, and secondary signs of obstruction. The significance of differences between the numbers of detected calcifications and the numbers of associated abnormalities on the scans obtained with varying section widths was tested with the McNemar test at a P level of less than .05. Spearman rho rank correlation coefficients were calculated to assess the correlation between the presence of uroliths and the presence of secondary signs.Uroliths were found in 72 of 147 (49.0\%) patients, and nephroliths were found in 16 patients (10.9\%). There was no significant difference between section widths of 1.5 and 3.0 mm with regard to the number of detected stones (264 uroliths and 61 nephroliths for both protocols). Transverse sections 5.0-mm wide revealed significantly fewer uroliths (n = 231; P < .001) and nephroliths (n = 47; P < .016). The final diagnosis was changed in four of 72 patients. All missed renal and ureteral stones measured less than 3 mm in diameter.Overlapping 3-mm sections are sufficient for the detection of urinary stone disease. Small calculi (<3 mm) may be missed on 5.0-mm-thick sections.