Children and adolescents with the high bone turnover comprise a high risk population for vitamin D insufficiency. A sample of 178 clinically healthy children aged 3 to 18 years who came from public schools and lived in North West of Greece participated in the study. They were grouped into three age groups (I: 3-10, II: 11-14 and III: 15-18 years of age). Blood samples were taken during winter and summer months for determining calciotropic hormones, calcium, phosphate and biochemical markers of bone synthesis.A high percentage (47%) of the subjects aged 15-18 years was found to have 25OHD <10 ng/ml in winter but much less (13-14%) of the younger ages (13-14 years), while in the summer they were all >10 ng/ml. The prevalence was even higher in the girls of the older group accompanied by lower Pi concentrations again in winter (win:1.19+/-0.03, sum:1.93+/-0.03 mmol/l, p < 0.001). The 24,25(OH)(2)D levels were changing in parallel to 25OHD, but again in the older subjects, during winter, they were by 2/3 lower than the summer ones (0.73+/-0.10 vs. 2.41+/-0.20 ng/ml, p < 0.001). No significant differences were found between seasons and groups in the 1,25(OH)(2)D levels. The biochemical markers of bone synthesis, osteocalcin (OC) and total alkaline phosphatase (ALP), were found significantly lower in the girls of the older group both in winter and summer respectively. Even in a sunny country like Greece the adolescents living in an urban area are in high risk for vitamin D deficiency during winter. Supplementation with vitamin D of milk, of popular beverages and perhaps some foods would be of help.