From 1973 through 1984, there were 138 cases of paralytic poliomyelitis reported in the United States; 105 (76%) were vaccine associated. Of the 105 vaccine-associated cases, 35 occurred in recipients of oral polio vaccine (OPV), 50 in contacts to OPV recipients, 14 in immune deficient individuals, and six in individuals who had no history of receiving OPV or contact with recent OPV recipients. Thirty-three (94%) of the recipient cases, 41 (82%) of the contact cases, and five (36%) of the immune deficient cases were associated with the first dose of OPV. The overall frequency of vaccine-associated poliomyelitis was one case per 2.6 million doses distributed. However, the relative frequency of paralysis associated with the first dose in the OPV series was one case per 520,000 doses vs one case per 12.3 million subsequent doses. Vaccine-associated paralytic poliomyelitis is rare and the risks of OPV are small. The greatest likelihood of paralysis occurs in association with the first dose of OPV and that likelihood is reduced in subsequent doses more for recipients than for their contacts.