Ciclesonide is a new-generation inhaled corticosteroid developed to treat the inflammation associated with persistent asthma. In order to identify the properties of ciclesonide responsible for anti-inflammatory activity, ciclesonide metabolism was investigated in human lung and liver precision-cut tissue slices. Three human lung and three human liver tissue slices were incubated with 25 μM [14C]-ciclesonide for 2, 6 and 24 h. Cellular viability was assessed using adenosine 5′-triphosphate content and protein synthesis in lung slices and adenosine 5′-triphosphate content and potassium retention in liver slices. Ciclesonide and ciclesonide metabolites were analysed in tissue samples using high-performance liquid chromatography with ultraviolet and radiochemical detection. Metabolite identity was confirmed using mass spectrometry. In lung slices, the inactive parent compound, ciclesonide, was initially converted to the active metabolite, desisobutyryl-ciclesonide, and subsequently converted to fatty acid conjugates. The reversible formation of fatty acid conjugates was a major pathway of ciclesonide metabolism in human lung slices. The primary conjugate was identified as desisobutyryl-ciclesonide oleate. Ciclesonide was metabolized to at least five polar metabolites in the liver. Dihydroxylated desisobutyryl-ciclesonide was the major polar metabolite in liver slices. Activation and fatty acid esterification in the lung followed by rapid inactivation in the liver may explain the improved safety profile and prolonged anti-inflammatory activity of ciclesonide. Copyright © 2006 John Wiley & Sons, Ltd.