Hunter disease is a mucopolysaccharidosis for which enzyme replacement therapy with idursulfase has been offered for 3 years. Therapy can be monitored by the urinary concentration of dermatansulfate and heparansulfate. There are no informations about breath analysis in these patients. A 38-year-old male person suffering from Hunter disease has been treated with idursulfase for a year, showing dramatic clinical improvement. Additionally to the monitoring of urinary excretion of metabolites the patient had breath analysis using MCC/IMS every week before enzyme replacement. The patient showed increasing concentrations of a peak P-1.5/0.547 which is related to acetone over the time. The concentration were higher than the concentrations of all patients ever tested with MCC/IMS. Furthermore a peak P-77.2/0.601 was found, which decreased after few weeks of therapy and needs further biochemical characterisation. These preliminary data show that breath analysis in patients with metabolic disorders might give further information about metabolism, especially under enzyme replacement therapy