Fibromuscular dysplasia (FMD) is a group of nonatherosclerotic, noninflammatory diseases of blood vessels with unknown aetiology. In our study, FMD was diagnosed in blood vessels in samples taken from kidneys, liver and lung of broiler chickens. The FMD occurred during rearing in 8 of 108 broiler chickens examined for the effects of intensive rearing on the.internal organs. Histopathological and immunohistochemical examinations revealed medial subtypes of FMD, medial fibromuscular dysplasia and medial fibromuscular stenosis. The first subtype presented as plugs in vessel lumens consisting of smooth muscle and fibrous connective tissue originating from the tunica media. The second subtype presented as a proliferation of smooth muscle cells and fibroblasts originating from the media and leading to lumen stenosis. The aetiology of FMD is still unknown. Thus, genetic factors are suspected as a cause of the disease. This is the first report of FMD in a vein of an animal species.