OBJECTIVE To determine the incidence of secondary signs of obstruction in patients with ureteral stones on unenhanced helical CT scans. METHODS Three hundred consecutive patients with suspected renal colic referred by the emergency department for an unenhanced helical CT scan were evaluated. One hundred and thirty two patients with ureteric calculi were viewed prospectively for the secondary signs of obstruction, which include hydronephrosis, hydroureter, perinephric stranding, nephromegaly, periureteral edema and difference in attenuation between acutely obstructed kidney and unobstructed kidney. RESULTS In the evaluated 132 patients, calculi were present in the proximal, mid and distal ureter in 12, 18 and 112 patients respectively. Four patients had a contralateral ureteric calculus while none had more than one ureteric calculus on the same site. Concurrent renal parenchymal calculi were seen in 40 patients with ureteric stones, hydronephrosis in 69, hydroureter in 93 and perinephric strandings in 66 patients. Nephromegaly was seen in 15 and periureteral edema in 42 patients. In 87% patients with acute renal obstruction, the affected kidney was less dense than the unobstructed kidney. Only 5 of 132 patients had no associated findings. CONCLUSIONS Unenhanced helical CT has rapidly become the imaging technique of choice in evaluating patients with acute ureteric colic. The secondary signs of obstruction provide supportive evidence of acute obstructive process.