Abstract: A 6-month-old boy developed transverse myelitis 7 days after the receipt of oral polio vaccine (OPV). A paediatric neurologist confirmed the diagnosis when the boy was aged 9 years. The boy had received his first scheduled OPV at the age of 4 months and had developed immunity to serotypes 1 and 2 but not to serotype 3. A poliovirus type 3 was isolated from stool and throat specimens collected from the boy in the first 2 days after symptom onset. This was shown, in a World Health Organization accredited laboratory, to be a vaccine strain by nucleic acid probe hybridiztion and enzyme-linked immunosorbent assay. The boy subsequently developed immunity to poliovirus serotype 3. It is accepted that poliovirus infection can present occasionally as transverse myelitis. This is estimated to occur in 1:125–1:800 cases. It is also accepted that OPV can cause vaccine-associated paralytic polio with a frequency of approximately one case per 2.5 million doses of OPV distributed. It seems feasible therefore that OPV could cause transverse myelitis with a frequency of 1 in 300 million to one in two billion doses distributed. In a 1993 report from the Institute of Medicine of the National Acadamies of the United States pertaining to vaccine safety, theoretical criteria were advanced for the establishment of a causal relationship between a vaccine and a clinical outcome. The clinical history and laboratory results in this case satisfy these criteria, providing plausible evidence for the causal link between OPV and transverse myelitis.