Time of day related changes in serum total IgE (and cortisol as marker rhythm) were documented in 6 non-allergic children (2 girls, 4 boys, 6 to 10 years old) and 3 boys (10 to 14 years) with allergic asthma but symptom free at the time of the study. Subjects were synchronized with a diurnal activity from 07.00 to 21.00 and a nocturnal rest. Venous blood was sampled at fixed times (07.30, 11.30, 16.30 and 22.30) during a 24 hours span for the healthy children and during a 48 hours span for the asthmatics. Radioimmunoassay methods were used for the determinations. Time series were analyzed according to conventional (t tested differences, ANOVA) and Cosinor methods. No IgE circadian rhythm was validated in healthy children while a large amplitude (approximately equal to 30% of the 24 hours mean) circadian rhythm with 2 diurnal peaks and a nocturnal trough was demonstrated (P less than 0.0023) in the asthmatics. Therefore, time qualified references are needed for the interpretation of total IgE as they are for many biological variables. Circadian rhythm of IgE is presumably related to those of lymphocyte subpopulations such as B, T and T-suppressor cells.