OBJECTIVE. The purpose of this study was to assess the effect of hydration status on renal medulla attenuation during unenhanced CT. SUBJECTS AND METHODS. An unenhanced CT scan was obtained in 10 healthy men after 12- 18 hr of complete fluid restriction and again after rehydration. CT images were reviewed for the presence and extent of dense medullar, defined as greater attenuation than the adjacent cortex. CT findings were correlated with urine-specific gravity and osmolality. RESULTS. Eight of the 10 subjects showed dense renal medulla on a CT scan after dehydration. After rehydration, complete resolution was seen on the CT scan in four of the eight subjects; three of the eight subjects showed a decrease in the extent of dense renal medulla. After rehydration, the eight subject showed a slight increase in the attenuation of the renal medulla on the CT scan. Dense renal medulla was seen significantly more often on dehydration images (p=.035). CONCLUSION. Increased renal medulla attenuation on a unenhanced CT scan can be a normal finding related to dehydration.