Using standard B scans, renal cortical echogenicity was graded by comparing the amplitude of echoes in the renal cortex with that of the adjacent liver, spleen, and renal sinus. Twenty-five consecutive patients were studied immediately before percutaneous renal biopsies. There was no correlation between the nature and severity of the glomerular lesion on renal biopsy and the sonographic findings. While the pyramids could be identified on sonography in all cases, the clarity of corticomedullary definition also did not correspond to any histopathologic finding. There was a definite relation, however, between the nature and severity of interstitial changes on biopsy and the echointensity of the cortex at sonography. Focal interstitial disease produced a minimal increase in cortical echogenicity. A greater increase was produced by diffuse scarring, and the most intense echogenicity of the cortex was seen in patients with active interstitial changes.