Prior studies using radiography have examined the relationship of ureteral stone size and location to the probability of spontaneous passage. Given the improved accuracy and new role of unenhanced CT in the diagnosis of acute ureterolithiasis, we studied the relationship of stone size and location as determined by unenhanced CT to the rate of spontaneous passage. Over a 29-month period, 850 patients with acute flank pain were evaluated with unenhanced CT. Confirmation of the CT diagnosis was obtained retrospectively for 172 patients with ureteral stones: 115 stones passed spontaneously and 57 required intervention. Stone size was defined as the maximum diameter within the plane of the axial CT section. Stone location was classified as proximal ureter (above the sacroiliac joints), mid ureter (overlying the sacroiliac joints), distal ureter (below the sacroiliac joints), and ureterovesical junction. The spontaneous passage rate for stones 1 mm in diameter was 87%; for stones 2-4 mm, 76%; for stones 5-7 mm, 60%; for stones 7-9 mm, 48%; and for stones larger than 9 mm, 25%. Spontaneous passage rate as a function of stone location was 48% for stones in the proximal ureter, 60% for mid ureteral stones, 75% for distal stones, and 79% for ureterovesical junction stones. The rate of spontaneous passage of ureteral stones does vary with stone size and location as determined by CT. These rates are similar to those previously published based on radiography.