PURPOSE: To evaluate the clinical significance of hyperechogenicity of the renal medulla and urinary bladder in normal neonates. MATERIALS AND METHODS: We investigated 31 clinically normal neonates, including one post-term, 16 pre-term, and 14 full-term babies, in whom hyperechogenicity of the renal medulla or urinary bladder was seen on ini-tial sonograms. All neonates underwent sonography while aged between 1 and 21 (mean: 2.5)days. For 14, fol-low-up sonography was performed 2-20 (mean : 6.8) days later. Eighteen neonates also underwent urinalysis, and two underwent a bacteriologic examination. RESULTS: Initial sonograms revealed (hyperechogenicity in the renal medulla (n=28) and urinary bladder (n=12). Twenty-five neonates were aged less than one week, four were aged 1-2 weeks, and two were aged 2-3 weeks. Urinalysis showed that six neonates were erythrocyte-positive (+/-:1, +1:4, +4:1), two were protein-pos-itive (+/-:2), and the others were negative. In all cases the results of bacteriologic study were negative. Follow-up sonography revealed that the hyperechogencity of renal medulla had regressed (n=12) or decreased (2), and that of the urinary bladder had regressed (n=4). CONCLUSION: In clinically normal neonates, hyperechogenicity of the renal medulla and urinary bladder was mostly visualized within the first week of life, and on follow-up sonography was seen to have regressed or decreased.