Purpose This study investigated the relationship between seasonal variations in 25-hydroxyvitamin D (25(OH) D) levels and growth in prepubertal children during both the pretreatment year and the first year of GH treatment. Methods The study included 249 short prepubertal children with a broad range of GH secretion, GH(max) during a 24 h profile median 23; range 1-127 mU/L, 191 boys (mean age +/- SD, 8.6 +/- 2.6 years), 58 girls (7.5 +/- 1.9 years) receiving GH treatment (mean 43 mu g/kg/day; range 17-99 mu g/kg/day). Serum 25(OH) D was measured using an automated IDS-iSYS immunoassay. Results 25(OH) D levels showed seasonal variation, and decreased significantly during GH treatment. 25(OH) D levels at start and first year reduction in 25(OH) D, correlated (-) with the first year growth response during treatment. The degree of GH secretion capacity within our study population of mainly non-GH deficient children and 25(OH) D sufficient (67 +/- 29 nmol/L) had no influence on 25(OH) D levels. Growth during GH treatment were independent of seasonal variations in 25(OH) D. Multiple regression analysis showed that 25(OH) D levels at treatment start, together with auxological data and IGF-binding protein-3(SDS), explained 61 % of the variation in first year gain in height(SDS). Conclusion 25(OH) D levels were associated with first year growth response to GH and may be a useful contribution to future growth prediction models.