Summary. Background: Vitamin D deficiency is a public healthcare issue and its correction is increasingly regarded as a cornerstone of preventive medicine. Methods: We designed a retrospective observational study to clearly define the burden of total vitamin D (25-hydroxyvitamin D) deficiency in a supposedly healthy population of outpatients residing in two regions (Emilia Romagna and Veneto) of Northern Italy. Results: 25-hydroxyvitamin D results were available for a total number of 5,096 outpatients in the two centers. The median value of 25-hydroxyvitamin D was 60 nmol/L, and was higher in women than in men (62 nmol/L versus 56 nmol/L; p