Our overall aim is to monitor iodine supply in a prospective study before and after the September 1998 increase of salt iodide content in Switzerland. Because iodide is supplied by alimentation, we moreover wondered whether urinary iodine concentration (UI) is governed by circadian rhythmicity. Forty-two subjects (18 males and 24 females, including 13 children) collected 3023 urine spots between May 1996 and May 1998, at a rate of three to five samples per month, at any time of the day. The results show that circadian rhythmicity of UI in adults and children was found independent of the individual subject, age, gender, and season. Lowest UI levels were found between 8-11 h. A curve increasing progressively between 12 and 24 h was obtained. UI returned to base-line levels between 21 and 22 h in children only. UI peaks occurred 4-5 h after main meals; children's peaks occurred later than that of adults. Although the existence of a circadian rhythm of UI is probably universal, its profile, however, depends on alimentation. Because nadir of UI is represented by morning spots, this might seem an appropriate collecting period. In view of the significant circadian rhythmicity of UI, studies with restriction of sampling time to morning hours, for example, cannot be directly compared with studies in which urine is sampled all over the day.