The objective was to investigate the validity of a self-monitoring device that estimates 24-h urinary salt excretion from overnight urine samples as a tool for education regarding salt restriction. Twenty healthy volunteers consumed test meals for 14 days, with salt content as follows: 10 g (days 1-5); 5 g (days 6-8, 12 and 14); and 13 g (days 9-11 and 13). On days 2-15, urinary salt excretion was estimated from overnight urine samples by a self-monitoring device. Twenty-four-hour urine samples were collected on days 5 and 8 to measure salt excretion directly. Blood pressure was measured in the morning and during sleep on days 1-15. Estimated urinary salt excretion measured by the device showed a correlation with salt intake, and the ratio of estimated urinary salt excretion to salt intake was 0.84 +/- 0.10 (days 2-6), 1.27 +/- 0.28 (days 7-9), 0.70 +/- 0.11 (days 10-12), 1.37 +/- 0.22 (day 13), 0.68 +/- 0.13 (day 14) and 1.33 +/- 0.19 (day 15). The correlation between estimated urinary salt excretion measured by a device and directly measured 24-h urinary salt excretion was significant (r = 0.65, P < 0.05) during the period of 10 g salt intake, but not during 5 g salt intake. Blood pressure in the morning was not influenced by the change in salt intake, but systolic pressure during sleep showed a significant increase or decrease according to the levels of salt intake. In conclusion, a self-monitoring device, which can estimate 24-h urinary salt excretion from overnight urine samples, is considered to be a practical tool for education regarding salt restriction, although a similar future investigation is needed in older and/or hypertensive subjects.