BACKGROUND Peripheral oxygen saturation (SpO(2)) measured by pulse oximetry is widely used in clinical practice, but its fluctuations over the course of the 24 h of a day have not been explored at length. Recently, we reported that children hospitalized due to non-cardiopulmonary causes had a circadian variation in SpO(2). This finding needed to be corroborated in healthy children, which is the objective of the present study. PATIENTS AND METHODS Healthy children residing in a state foster home were studied with pulse oximetry every 2h for 24h. RESULTS Eighty two children were included in the study, ranging in age from one month to 6.5 years (average ± standard error of 3.06 ± 0.16 years), with a weight-for-length/height percentile of 65.5 ± 2.9. In 65 (79.3%) children, the SpO(2) levels followed a sinusoidal curve suggesting circadian rhythm. The total group of sinusoidal curves in this population had a mesor of 95.10 ± 0.08% SpO(2), period of 21.05 ± 0.54 h (in 53.8% of these children, the period was between 20 and 28 h). The maximum SpO(2) was reached at 3:14 pm ± 16 min, and the minimum at 5:16 am ± 48 min. When the 24 h were divided into four periods, it was demonstrated that the highest SpO(2) levels were reached between 2 pm and 8 pm. CONCLUSIONS In this population of clinically healthy children, there was a circadian variation in pulse oximetry, with maximum values in the late afternoon and minimal values in the early morning.