Background: Multiple studies associate low vitamin D levels with cancer morbidity and mortality. However, few studies have measured vitamin D in pediatric patients with malignancy. Our aim was to assess vitamin D status in a large cohort of pediatric patients with cancer and to define risk factors for deficiency. Methods: Circulating 25-hydroxyvitamin D (25OHD) levels were measured in 211 patients. Calcium intake and sun exposure habits were assessed in 142 patients (age 12.1 ± 5.8 y; number of male patients, 69; mean time from diagnosis, 4.4 ± 3.8 y).Results:Daily calcium intake was 66.2 ± 39.3% of the recommended daily allowance. Mean 25OHD levels were 20.6 ± 7.9 ng/ml. Vitamin D deficiency (15 ng/ml) was found in 24.6% of the patients and insufficiency (15-20 ng/ml) in 23.2%. Younger age and amount of sun exposure were associated with higher serum 25OHD. No association was found with calcium intake, disease type, gender, BMI SD score, years since diagnosis, or stem cell transplantation. The 25OHD levels during winter were significantly lower than the summer levels.Conclusion:The prevalence of vitamin D deficiency and insufficiency in pediatric patients with a history of malignancy was high, whereas calcium intake was low. These findings are concerning, given the risk for osteoporosis in this population and the possible role of vitamin D in the context of malignancy.