Atopic dermatitis (AD) is a chronic relapsing inflammatory skin disease [1]. It affects 10%-20% of children, the most prevalent form being chronic disease, and 1%-3% of adults, particularly in the most developed Western societies [1][2][3]. Fifty percent of cases resolve during adolescence, with up to 20% of cases persisting into adulthood and a prevalence of 1%-3% in patients aged more than 60 years [4].Available evidence on the prevalence of severe AD is uncertain and scarce at the population level. No studies on the prevalence of AD have focused on Spain. The main goal of this study was to describe the prevalence of severe AD in routine clinical practice at different locations in Spain. As a secondary goal, associated comorbidities and therapeutic approaches were described.A cross-sectional (prevalence) study was conducted based on a review of medical registries (digital databases, dissociated data). The study population was obtained from health care providers' registries, which were unified in the dissociated database of the RedISS (Red de Investigación en Servicios Sanitarios) Foundation. Data came from the OMIap digital clinical history and other complementary databases. Registries from patients in Asturias, Catalonia, and the Balearic Islands were obtained. Patients who required healthcare services during 2015-2016 were included. The inclusion criteria were age ≥18 years, registered diagnosis of severe AD at least 12 months before the study, inclusion in the prescription program (registered daily dose, timing and duration of each treatment; ≥2 prescriptions during the follow-up period), and ability to guarantee regular follow-up (≥2 health registries in the system; at least 1 visit to the dermatology service). The exclusion criteria were change in health center or move outside the study areas, permanent hospitalization, and history of seborrheic dermatitis, contact dermatitis, and/or dyshidrotic eczema.Registries of patients with AD were obtained from the International Classification of Primary Care in the European Union [5] (codes S87, S88) and/or the International Classification of Disease (Ninth Edition), Clinical Modification (ICD-9-CM; 691.8, 692.9, 706.8), which include AD, allergic dermatitis, allergic eczema, atopic eczema, atopic neurodermatitis, and xerosis. AD was diagnosed by the physician (reference dermatologist) according to the criteria of Hanifin and Rajka [6] (AD present for over 1 year). AD was considered severe when the patient received an immunosuppressant (cyclosporine, methotrexate, azathioprine, mycophenolate mofetil) or a biologic (omalizumab, rituximab) during followup or had been hospitalized because of AD. Furthermore, administration of systemic corticosteroids and other treatments (immunoglobulins, interferon g, hydroxychloroquine, alefacept) received by the patient was registered. Data were obtained from registries of pharmaceutical dispensation and hospital pharmacy services (Anatomical Therapeutic Chemical Classification System) [7]. Age (continuous and by range), sex, and ...