Patients (4) with decreasing renal function evaluated by sonography had features characteristic of medullary nephrocalcinosis. In each of these cases the cortical echogenicity was normal while focal areas of increased echogenicity corresponding to the renal pyramids were seen. In 3 of the 4 cases, medullary nephrocalcinosis, evident on sonography, was not observed even in retrospect on abdominal radiographs.