Background:To date, few studies have investigated serum vitamin D status in patients with inherited ichthyosis.The aim of this study was to determine the prevalence of vitamin D deficiency (defined as serum level <10 ng/mL)in a French cohort of patients and to identify associated risk factors.Methods:This was a prospective observational study performed in a hospital reference center with expertise forrare skin diseases. Patients’clinical characteristics were recorded. Serum concentration of 25-hydroxyvitamin D andparathyroid hormone were determined. For patients with vitamin D deficiency, serum calcium, serum phosphorusand bone mineral density were also investigated. Comparisons between groups (25-hydroxyvitamin D <10 ng/mLversus≥10 ng/mL) were conducted by univariate and multivariate logistic regression.Results:Of the 53 included patients, 47 (88.7%) had serum 25-hydroxyvitamin D below the optimal level of 30 ng/mL:18 (34%) had vitamin D sufficiency, 14 (26.4%) had vitamin D insufficiency, and 15 (28.3%) had vitamin D deficiency. Anegative linear correlation was found between 25-hydroxyvitamin D and parathyroid hormone levels for the wholestudy population. Serum calcium and phosphorus levels were normal for the 15 patients with vitamin D deficiency.Bone mineral density was investigated for 11 of these latter 15 patients, and six of them had osteopenia. Winter/springseasons of vitamin D measurement, severity of ichthyosis, and phototypes IV–VI were identified as independent riskfactors for vitamin D deficiency.Conclusions:Clinicians should be aware of the risk of vitamin D deficiency in the management of patients withinherited ichthyosis, especially in winter and spring, and in case of dark skin or severe disease.