Renal sonography is frequently requested in patients with an elevated or rising creatinine, often with the instruction to "rule out obstruction." A prospective study of patients undergoing renal sonography for newly discovered azotemia (creatinine greater than 1.8 mg/dl) or worsening renal failure was undertaken to evaluate the clinical utility of the sonographic examination. Patients with known obstructive uropathy, renal calculi, renal mass, or hematuria were excluded. In 189 patients, 17 (9%) had hydronephrosis: 11 unilateral (one with a solitary kidney) and six bilateral. Obstructive causes were subsequently established in the solitary kidney and in four patients with bilateral dilatation. These five patients (2.6%) with clinically significant hydronephrosis were shown subsequently to have a clinical history that strongly suggested the presence of urinary tract obstruction. Our data suggest that, in patients without a clinical history that suggests obstruction (such as calculi, bladder outlet obstruction, or pelvic mass), the likelihood of finding bilateral hydronephrosis by sonography is small.