To assess whether ultrasonography (US) with or without plain abdominal radiography (kidney, ureter, bladder [KUB] radiography) can replace intravenous urography (IVU) in detection of acute urinary tract obstruction, 101 consecutive patients with renal colic were evaluated with US followed immediately by IVU. Receiver operating characteristic (ROC) curves for US diagnosis of acute urinary tract obstruction yielded sensitivities of 91% and 92% for two reviewers at a specificity of 90%. There was no statistically significant difference between US and IVU results. When US was combined with KUB radiography, ROC curves yielded sensitivities of 94% and 97% for two reviewers at a specificity of 90%.KUB radiography alone was of limited diagnostic value. For US alone, no false-positive results occurred, and the false-negative results (n = 9 and n = 6 for two reviewers) were encountered in cases of grade 1 hydronephrosis and nondilated obstructive uropathy. The authors conclude that US combined with KUB radiography can replace IVU in initial evaluation and follow-up of the great majority of patients with renal colic.