We assessed to what extent sodium intake can be estimated from the sodium content of a spot-urine specimen (spot-UNa) collected within 4 hours after the first voiding upon awakening but before breakfast. Subjects were asked to collect spot-urine and 24-hour urine specimens other than the spot-urine for a 3-day period, either successively or intermittently. The coefficient of correlation between spot-UNa and urinary excretion of sodium in 24 hours (24-h UNa) collected on the first day was 0.47. When the coefficient of the variation of creatinine excretion in the spot-urine was above 20%, the sample was discarded as the "outlier", and in this case the coefficient of correlation was 0.725. A marked fluctuation of creatinine excretion in spot-urine was considered to represent technical errors at the time of the spot-urine collection. The coefficient of variation of intra-individual 24-h UNa exceeded 20%, suggesting that a single determination 24-h UNa does not represent the individual average of daily urinary excretion of sodium. It is concluded that the determination of a substantial number of spot-urine specimens to estimate daily salt ingestion of a given subject may be more reliable than a single determination of 24-h UNa, if the "outlier" of creatinine excretion in spot-urine specimen is excluded from the determination of spot-UNa. The average urinary excretion of sodium in 24 hours by the Japanese subjects investigated here was 218±67 mEq.