In the autumn of 1975 five cases of poliomyelitis occurred within 6 weeks in German children of families of low socioeconomic class. They lived in two districts of Freiburg with close familial and occupational contacts. One child had been immunised against polio once orally several years ago and none of the others were immunised. The clinical course and results of investigations of the environment are reported. Included is the state of immunisation of 472 school beginners in Freiburg schools and the antibody levels of 284 children aged 1 to 10 years from Freiburg and the adjoining areas.