A case-control study was conducted between November 1992 and May, 1993 to assess the effectiveness of routine oral polio vaccine (OPV) immunization against paralytic poliomyelitis among children aged 1-5 years in Cairo city. Cases of paralytic poliomyelitis at Imbaba Institute of poliomyelitis and Ain-Shams Pediatric Hospital fulfilling the study criteria were obtained. The diagnosis of cases was based on clinical grounds. Age and sex-matched controls were recruited concurrently from patients admitted for reasons other than poliomyelitis. There were 55 cases of poliomyelitis. Vaccination status of cases and controls was ascertained by asking the mothers through interviewing using a questionnaire that had been developed for the purpose of the study. Vaccine efficacy (VE) was 86% (95% CI: 39-97%) and 92% (95% CI: 64-98%) for 3 and 4 doses of OPV respectively. For a vaccine like OPV with the potential of near 100% VE after 3 doses, the estimated level of protection (86%) is considered low especially when we realize the enormous efforts carried out by Ministry of Health and UNICEF in Egypt for upgrading the cold-chain. Higher figures of VE are supposed to be achieved if we are going to meet the challenge of poliomyelitis eradication by the year 1994. Potential biases in study design were also discussed.