OBJECTIVES: The aim of the study was to determine the prevalence of vitamin D deficiency and identify the relation between 25-hydroxyvitamin D (25-(OH)D) levels and the consumption of dietary sources of vitamin D or exposure to anticonvulsants in girls and women with Rett syndrome (RTT). SUBJECTS AND METHODS: Retrospective review of the medical records of 284 girls and women with RTT to determine serum 25-(OH)D and parathyroid hormone levels, nutritional status, dietary sources of vitamin D, exposure to anticonvulsants, degree of mobility, and MECP2 status. RESULTS: Twenty percent of girls and women who were tested (n=157) had 25-(OH)D levels <50nmol/L. Multivitamin supplements, vitamin D-fortified milk, and commercial formulas were consumed by 40%, 52%, and 54%, respectively. Anticonvulsants were used by 57%, and 39% ambulated independently. Median 25-(OH)D levels were lower in individuals who did not receive multivitamin supplements (P<0.05) or commercial formulas (P<0.001) than in those who did. Median 25-(OH)D levels differed (P<0.01) among racial and ethnic groups, but the number in some groups was small. Nutritional status, use of anticonvulsants, degree of mobility, and MECP2 status did not influence 25-(OH)D levels. CONCLUSIONS: Vitamin D deficiency is prevalent in girls and women with RTT. The use of multivitamin supplements or commercial formulas is associated with improved vitamin D levels. Attention to vitamin D may enhance bone mineral deposition and reduce the frequency of bone fractures in these individuals. Copyright © 2011 by ESPGHAN and NASPGHAN.