In May-July 2004, type 1 vaccine-derived poliovirus (VDPV) was isolated from 2 case patients with polio and a contact of a third case patient in Guizhou Province.We conducted a field investigation of the outbreak, characterized outbreak isolates, and retrospectively reviewed national polio surveillance data for other VDPVs.Case patients were unimmunized children, 0.9-3.2 years old, living in 2 villages 40 km apart. Immunization coverage in the affected villages was very low. Isolates differed from the Sabin 1 type by 9-11 VP1 nucleotides (1.0%-1.2%); which indicated, on the basis of known rates of mutation of Sabin strains, that they had been circulating for 50,000 children with acute flaccid paralysis and their contacts; 8 (80%) were found in southern provinces, and 9 (90%) spontaneously disappeared.This is the first polio outbreak in China in over a decade and the first due to VDPV. The short duration of circulation demonstrates the rapidity with which attenuated Sabin strains can revert to a wild phenotype. One to two VDPVs have been identified each year, primarily in densely populated subtropical regions of southern China. This outbreak highlights the need to consider risks of paralysis from vaccine-derived strains in development of national poliomyelitis immunization policy.