Seasonal variations in urinary calcium excretion have been observed in adults and are thought to be due to seasonal changes in sunlight exposure, which results in fluctuation of blood levels of 25-hydroxyvitamin D. The goal of the present study was to explore the possible effect of season on urinary calcium/creatinine ratio (UCa/Cr) in healthy school children. We studied 65 healthy Caucasian children aged 5.1-14.6 years (mean+/-SD 9.6+/-2.0, median 9.7 years). Random, non-fasting urine samples were collected from each participant between the 1st and 5th days of the month for 16 successive months and analyzed for UCa/Cr. In parallel, meteorological data, which included average monthly temperature and average monthly UV index, were collected for the whole study period. A mathematical model describing the monthly mean values of UCa/Cr as a sine function was used to determine the presence of seasonal variation. The observed maximum mean UCa/Cr, 0.092 mg/mg (0.260 mmol/mmol), occurred in the late summer months (August/September) and minimum mean UCa/Cr, 0.083 mg/mg (0.235 mmol/mmol), occurred in mid-winter (January/February), reflecting an infinitesimal and statistically insignificant change. Moreover, both values were well within the previously established normal range for UCa/Cr of </=0.20 mg/mg (0.566 mmol/mmol). The pattern seen in the last 4 months of the study repeated that seen in the first 4 months. The mean temperature in July of the 1st year was 27.1+/-2.6, January 1.8+/-5.4, and July of the 2nd year 27.0+/-2.2 degrees C. The mean UV index in July of the 1st year (7.61+/-1.01) was not different from that in July of the 2nd year (7.73+/-0.86), and both were statistically much higher than that in January (1.37+/-0.44) ( P<0.001). Evidently, in spite of significant changes in seasonal meteorological indices of sunlight, no significant seasonal variations in UCa/Cr were observed in a cohort of healthy school children. Thus the current UCa/Cr value used to diagnose hypercalciuria does not require a seasonal adjustment.