The authors have described the first cases of Swollen Head Syndrome observed in Poland in 1991—1992. The clinical signs, pathomorphological findings and treatment were described. Virus isolated from one case was classified on the basis of viral and biochemical tests and electron microscopy as Pneumovirus of Paramyxoviridae. In sera of chickens, coming from the flock affected with the disease, specific antibodies against Swollen Head Syndrome antigen were found by the ELISA test.