Abstract: [Abstract] Objective：To investigate the presence of red bacterial complex (Porphyromonas gingivalis (Pg) , Tannerella forsythus (Tf) and Treponema denticola (Td)) in pulp tissue with retrograde pulpitis. Methods：Pulp tissue was collected from 40 teeth with retrograde pulpitis. Pg, Tf, Td and the total bacteria in the samples were assayed respectively using SYBR Green real-time PCR. Results Pg, Tf and Td were detected in 22.5%, 82.5 and 47.5% of the samples respectively. There is a synergistic action among Pg, Tf and Td (OR=2.83（1.26~6.38）). Conclusions：The red complex was related to the infection in retrograde pulpitis.