OBJECTIVES: Asthma is a major public health problem with considerable economic impact. The aim of this study was to asses the extent of uncontrolled disease and associated medical costs in severe asthma. METHODS: Data were obtained from the PHARMO Record Linkage System (PHARMO RLS) which includes drug dispensing and hospitalizations for 2 million subjects in The Netherlands. Severe asthma patients (12–49 years) were defined as those who had used long-acting beta-agonists and inhaled corticosteroids for over 200 days and short-acting beta agonists for at least 100 days in 2004. Uncontrolled asthma was defined as a hospitalization for asthma or use of multiple, short (<30 day) courses of oral corticosteroids. Reimbursed costs of asthma drugs and hospitalizations were calculated. A matched case-control analyses was performed to compare asthma treatment of cases (hospitalized for asthma or multiple courses of oral corticosteroids) and controls. RESULTS: A total of 4.7% of patients receiving at least 2 asthma preparations in the previous year had severe asthma. A total of 17.4% of these were defined as having uncontrolled asthma. Excess drug costs for uncontrolled asthma with hospitalization were €664 per patient per year and €282 per patient per year for patients without hospitalization. Including hospital admission costs, excess costs were over €10,000 per patient per year. Lack of control did not seem to be caused by under-treatment. CONCLUSION: Poor control of severe asthma leads to disproportionately increased direct costs compared to controlled severe asthma, especially when hospital admission is required. Hospitalized patients representing 2% of the patients with severe asthma are responsible for 23% of the total direct costs.