In a seroepidemiological study performed to investigate immunity against poliomyelitis in a population of drug addicts in a rehabilitative residential centre we demonstrated a widespread lack of poliovirus neutralizing antibodies. This was more evident in the HIV positive subjects, 27% of whom were seronegative for poliovirus type 1, 27% for type 2 and 34% for type 3, with 11% seronegative for all three types. These results indicate a gap in immunity to poliomyelitis in the examined population.