In the past, cross-sectional studies of small groups have not always shown a significant relation between habitual sodium intake and blood pressure (BP). This study examined the relation between 24-h urinary sodium excretion (Salt24, g/day) and BP during sleep in young subjects. The subjects were 132 medical students (93 males, 39 females) aged 21 to 26 years, with no renal disorder. Urine was collected for 24 h, and sodium (Na), chloride (Cl), and creatinine (Cr) excretion were measured. The 24-h urinary Cr excretion (Cr24) was compared with the Cr24 estimated from lean body mass as a guide to ensure accurate collection of 24-h urine. The natural logarithm [ln (Salt24/Cr24)] was taken as an index of sodium intake. On the same day, a portable multi-biomedical recorder (TM2425) was used to measure 24-h systolic BP (SBP)/diastolic BP (DBP). Daytime BP was calculated as the average of waking-time BP, and sleep BP was taken as statistical base BP (minimum BP) during nighttime sleep. No correlation was observed between ln (Salt24/Cr24) and daytime BP. The correlation coefficient with sleep SBP, on the other hand, was 0.34 (p<0.05) in the male group and 0.54 (p<0.01) in the male high-BP group (24-h BP>127/75 mmHg, n=28). No relation of this kind was observed in the female group. In conclusion, despite the small number of subjects, the present cross-sectional study revealed a significant positive correlation between sodium intake and sleep SBP in young men with high BP.