Renal sonography was performed in 36 patients with clinical and/or laboratory evidence of AIDS, usually because of deteriorating renal function. In 15 patients, histopathologic specimens were reviewed to characterize renal pathologic changes underlying the sonographic findings. Sonographic evaluation included determination of renal size and cortical echogenicity according to a standard grading system. Pathologic specimens were evaluated for tubular and glomerular abnormalities. Sonography showed either normal-sized or enlarged kidneys with grade I cortical echogenicity in 13 patients (36%), grade II in three patients (8%), and grade III in five patients (14%). Fifteen patients (42%) had normal renal echogenicity. In addition to focal segmental glomerulosclerosis the pathologic examination showed different degrees of tubular abnormalities. Striking, irregularly dilated, infolded tubules with flattened epithelium and intratubular deposits of proteinaceous material, and sometimes cystlike formation involving the medulla and cortex, were seen in two patients with grade III kidneys, and mild dilatation of the tubules was seen in four patients with grade I disease. Moderate tubular dilatation was observed in one patient with grade I disease. No significant tubular abnormality was seen in one patient with grade I disease or in seven patients with normal renal echogenicity. Although glomerular changes contribute to the increase in renal echogenicity, we postulate that the main factors responsible for the increased echogenicity in AIDS nephropathy are the striking tubular abnormalities seen in these patients.