Abstract: Objective: To investigate the effects of one-time root canal therapy in the treatment elderly pulp necrosis complicated with apical periodontitis.Methods: One hundred and thirty-eight elderly patients with pulp necrosis complicated with apical periodontitis were randomly divided into the observation group(n=71) and control group(n=67).Two groups were treated with machine Protaper NiTi files combined with ultrasonic washing the root canal.The observation group were treated with one-time root canal filling,and the control group were treated with secondary root canal filling.The pains in two groups were assessed before treatment and 7 d after operation,and the clinical effects in two groups were assessed 7 d,6 months,12 months and 24 months after operation.Results: The difference of the total effective rate between the observation group(91.55%) and control group(92.54%)7 d after operation was not statistically significant(P>0.05).The difference of the incidence rate of postoperative teeth pain between the observation group(11.27%) and control group(7.46%) was not statistically significant(P>0.05).The success rates in observation group 6 months,12 months and 24 months after operation were 74.19%,76.27% and 79.63%,respectively,and those in control group were 71.43%,72.41% and 68.09%,respectively.The differences of the success rates in postoperative different time-points between two groups were not statistically significant(P>0.05).Conclusions: The clinical effect of one-time root canal treatment in the treatment elderly pulp necrosis complicated with apical periodontitis is equivalent with the secondary root canal treatment,which can help to reduce the number of visits and shorten the course of treatment,and is safe and effective.