An 8-month-old girld develoepd paralytic poliomyelitis. Twenty-nine days prior to the onset of paralysis she had been in contact with a sibling recently immunized with trivalent oral polio vaccine. The patient died because of intercurrent respiratory infection. "Vaccine-like" Type 3 poliovirus was isolated from the throat, stool, and postmortem brain and spinal cord spciemens. Histologic examination revealed depltion of all lymphoid tissues. Serum immunoglobulin levels were adequate. We believe this child presents a variant of the syndrome of hereditary thymic dysplasia.