This study was conducted to determine the frequency of vitamin D deficiency and its correlation with different factors. Three hundred and thirteen healthy children and adolescents (192 females and 121 males aged 8-18 years, mean +/- SD, 12.7 +/- 2.3 years) were enrolled, and measurements of serum 25-hydroxyvitamin D [25(OH)D] (using EIA) and intact parathyroid hormone (iPTH) (using immunoradiometric assay (IRMA)) were conducted. The grades of vitamin D status were defined according to blood level of 25(OH)D as follows: severely deficient < 12.5; deficient, > or = 12.5 and < 25; insufficient, > or = 25 and < 50; normal > or = 50 and < 250 nmol/L. Severe deficiency was detected in 25% of subjects (males 8%; females 92%), deficiency in 27% (males 34%; females 66%) and insufficiency in 26% (males 58%; females 42%). The mean 25(OH)D level in males was significantly greater than that in females (p < 0.001), and this level was significantly higher in prepubertal compared to pubertal subjects (p < 0.001). 25(OH)D had a negative correlation with iPTH (p < 0.001). The curve of iPTH began to rise when 25(OH)D reached 75 nmol/L. The level of 25(OH)D had a negative correlation with BMI-SDS and height-SDS in females (p-value, 0.01 and 0.039, respectively). The subjects did not have any signs or symptoms of rickets. Frequency of vitamin D deficiency did not have any significant seasonal variation. Furthermore, vitamin D deficiency was not found to be related to the type or location of the subjects' homes. In this study, subclinical vitamin D deficiency was significantly more prevalent in females, particularly those undergoing puberty. Children who were obese and taller than average, had lower levels of 25(OH)D, and level of 25(OH)D should be maintained > 75 nmol/L in order to prevent PTH rising.