Pig endocardiosis is a pathological process affecting cardiac valves that is characterised by the accumulation of glycosaminoglycans (GAG) in the extracellular matrix. To investigate the involvement of GAG in the condition, the morphology of the mitral valves from 23 affected pigs and seven normal controls was studied and qualitative and quantitative biochemical analyses of GAG were made. Gross and histopathological lesions were characterised by valve enlargement, collagen disorganisation and myxoid degeneration. No differences between normal and diseased valves were detected by lectin histochemistry. Electron microscopy revealed myofibroblast differentiation of many fibroblasts. A statistically significant increase of total GAG and hyaluronan was detected in the mitral valves of the pigs with endocardiosis by spectrophotometric, electrophoretic and densitometric analysis of the extracted GAG. Although it is not known whether the change in hyaluronan is a primary event or a result of other changes in the extracellular matrix, its accumulation in association with myofibroblast differentiation suggests that it plays a pathogenetic role in pig endocardiosis.