Objective To investigated the antimicrobial effect of photo-activated disinfection therapy（PAD） in association with endodontic treatment in vitro and in vivo studies. Methods In vitro study, 30 single-rooted teeth were selected and contaminated with Enterococcus faecalis ATCC29212. The specimens were randomly divided into 3 groups. PAD therapy, 2.5% NaClO irrigation（positive control） and saline solution（negative control） were used to disinfect the root canals in each group respectively. In vivo study, 50 single-root canals teeth were selected, and then randomly distributed into 2 groups. Each root canal was instrumented and irrigated with 2.5% NaClO. Then the root canals in group 1 were disinfected with PAD and the root canals in group 2 were irrigated with saline solution. Microbial sample was taken before and after instrumentation/irrigation and PAD therapy, plated onto the BHI agar and the colony forming units were counted. Results In vitro study, the bacterial counts reduced 99.86% after PAD disinfection compared with 100% in positive control group and 96.94% in negative control group. The recovery of bacteria was detected in 10 samples after PAD disinfection compared 6 samples in positive control group and 10 samples in negative control group. In vivo study, bacterial counts reduced 99.98% after instrumentation in two groups. After the PAD therapy, there was no positive colony forming units were detected in group 1, compared 36% ruduction in group 2. But no significantly difference was found between the two groups（P>0.05）. Conclusion PAD can decrease Enterococcus faecalis load in root canals, which is lower than NaClO. The use of PAD added to endodontic treatment leads to an enhanced decrease of bacterial load in infected root canals.
