Objectives: Over the past few years researchers have suggested that vitamin D plays a diverse role in autoimmune diseases such as systemic lupus erythematosus (SLE). We sought to determine the prevalence and predictors of vitamin D deficiency in a cohort of non-supplemented female SLE patients from the Mediterranean region. Methods: We carried out a prospective cohort study on all SLE patients who had visited the Department of Rheumatology at the Parc de Salut MAR (Barcelona, Spain) between June 2007 and December 2008, excluding those who had been taking vitamin D supplements (total: 73 patients, all female). For each patient, demographic information was collected; scores were measured for disease severity [SLE Disease Activity Index (SLEDAI)] and structural damage [Systemic Lupus International Collaborating Clinic/American College of Rheumatology, (SLICC/ACR) Damage Index]; pharmacological treatment was recorded; analytical variables were analysed; and plasma levels of 25-hydroxy vitamin D [25(OH)D] were quantified. Results: Among the patients in our cohort, 68.5% [95% confidence interval (CI) 60.3-79.2] exhibited vitamin D deficiency [plasma level of 25(OH)D < 30 ng/mL]. The predictors for vitamin D deficiency were daily sunscreen use [odds ratio (OR) 1.67, p = 0.02] and high body mass index (BMI) (OR 1.32 when adjusted for seasons and patient age, p = 0.04). We did not find any correlation between vitamin D deficiency and SLEDAI score (p = 0.31), SLICC/ACR score (p = 0.82), or any other of the variables. Conclusions: Vitamin D insufficiency is highly prevalent among SLE patients, even in southern regions. Sunscreen use and obesity increase the risk. Clinicians should be aware of these factors and supplement SLE patients at risk of vitamin D deficiency accordingly. © 2012 Taylor & Francis on license from Scandinavian Rheumatology Research Foundation.