OBJECTIVES: Severe persistent asthma is not always adequately controlled and its prevalence is unknown among pneumology and allergy hospital units. The aim of this study is to determine uncontrolled severe persistent asthma prevalence among asthmatic patients attended in hospital units, to describe their clinical characteristics and to determine the prevalence of sensibility to common aeroallergens. METHODS: Cross-sectional epidemiologic study conducted in 201 hospital pneumology and allergology units in Spain. Asthmatic patients attending these services (in and out-patients) during a 6-month period were registered. Demographic data, clinical characteristics, skin-prick test, total serum IgE, asthma control test (ACQ) and quality of life (AQLQ) assessments were collected among a sample of asthmatics patients. RESULTS: A total of 1,423 (3.9% [CI 95%: 3.7-4.1%]) out of 36,649 asthma patients attending hospital units, had uncontrolled severe disease, data from 330 was collected. They showed a poor asthma control (a mean (SD) ACQ score 4.17 [ 0.96 ]). 55.9% of the sample had a positive skin-prick test to common aeroallergens and 53.6% showed high levels of total serum IgE. The percentage agreement between control assessment by the investigator and by GINA guidelines was moderate (65.3%, Kappa=0.365), this discrepancy is basically explained by an overestimation of control by specialists. CONCLUSION: 1.) Uncontrolled severe persistent asthma shows a limited prevalence among hospital asthma patients in Spain; 2.) The level of the asthma control is overestimated by physicians; and 3.) A half of the sample fulfilled criteria of the allergic disease.