PURPOSE: To evaluate the clinical significance of transient hyperechogenicity of the renal medulla in neonates by comparing the clinical features, urinalysis and follow-up ultrasonographic examination of the control group. MATERIALS and METHODS: One hundred ten neonates were divided into 2 groups, hyperechoic and normal renal medulla groups, and all of them underwent abdominal ultrasound with a 7.5MHz linear transducer (Sonoace 8800MT, Medison, Korea) from November 1999 to January 2000. Whether there was any difference in clinical features including birth weight, height, body surface area, gestational age, sex, date of examination and mode of delivery between two groups was evaluated. In addition, any difference in their urinary osmolarity, albumin, uric acid, and calcium in 41 neonates who underwent urinalysis was evaluated. In ten neonates with hyperechoic renal medulla underwent follow-up study, the follow-up ultrasonographic findings were compared with the initial study. RESULTS: In 67 of 110 (61%) neonates, ultrasonography demonstrated hyperechoic renal medulla. There was no difference in clinical features between the hyperechoic renal medullary group and normal group. In 41 neonates, there was no significant difference in urinalysis between two groups. (Osmolarity=146.46+/-68.4 mOsml/KgH2O in the hyperechoic renal medullary group vs. 149.8+/-77.7 mOsml/KgH2O in the normal group; albumin=13.9+/-10.2 mg/ml vs. 17.6+/-13.6 mg/dl; uric acid=50.0+/-23.3 mg/dl vs. 44.9+/-34.1 mg/dl; calcium= 1.38+/-3.0 mg/dl vs. 0.44+/-0.07 mg/dl.) Ten neonates who underwent follow-up ultrasonography within 20 days after the initial study showed the normal medullary echogenicity. CONCLUSION: There were no significant difference between the hyperechoic renal medullary group and the normal echogenic group in their clinical features and urinalysis. Therefore, the hyperechoic renal medulla in neonate is considered as an usual and transient finding which disappears on follow-up study.