OBJECTIVE: Asthma is a major public health problem with considerable economic impact. The highest costs being observed in patients with severe asthma. Furthermore, despite the use of recommended therapies, asthma control can still be poor. Therefor, the objective of this study was to assess the extent of uncontrolled disease and associated medical costs in severe asthma. METHODS: The PHARMO Record Linkage System includes among others drug dispensing and hospitalizations for > or = 2 million subjects in The Netherlands. Severe asthma patients used long-acting beta-agonists and inhaled corticosteroids for over 200 days and short-acting beta-agonists for at least 100 days in 2004. Severe uncontrolled asthma was defined as a hospitalization for asthma or use of multiple short courses of oral corticosteroids assessed in 2004. Reimbursed costs of asthma drugs and hospitalizations were calculated during this year. A matched nested-case control study was performed to identify treatment-related risk factors for uncontrolled disease. Information on clinical diagnosis of (severity of) asthma was not available. RESULTS: About 17% of patients with severe asthma aged 12-49 years (N = 1158) showed lack of control. Excess drug costs for severe uncontrolled asthma with hospitalization mounted up to 700 Euro per patient per year and 300 Euro per patient per year for patients without hospitalization. Including hospital admission costs, excess costs mounted up to over 10,000 Euro 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 severe controlled asthma, especially when hospital admission is required.