Objective: To evaluate the feasibility of full-mouth debridement ( subgingival scaling and root planning , SRP) by 2 times within 1 week and compare the clinical effects of different sequences of debridement-antibiotic usage in patients with severe chronic periodontitis ( CP ) .Methods: A double-blinded, placebo-controlled, randomized clinical trial was conducted in 30 severe CP patients (14 males and 16 females, 40.5 ±8.4 years old on average from 35 to 60 ) receiving 3 different sequences of debridement-antibiotictherapy:Group A, antibiotic usage (metronidazole, MTZ, 0.2 g, tid, 7 d;amo-xicillin, AMX 0.5 g, tid, 7 d) was started together with SRP ( completed by 2 times in 7 d);Group B, antibiotic usage (MTZ 0.2 g, tid, 7 d;AMX 0.5 g, tid, 7 d) was started 1 d after SRP(completed by 2 times in 7 d);Group C, SRP alone[probing depth (PD), bleeding index (BI) and tooth mobility] was examined .The average full-mouth probing depth , the average full-mouth proximal probing depth ( pPD) , the percentage of sites with PD >5 mm ( PD>5 mm%) , the percentage of sites with proximal PD>5 mm ( pPD>5 mm%) , the average bleeding index ( BI) and the percentage of sites with bleeding on probing ( BOP%) were calculated .Clinical examinations were performed at baseline and 2 months post therapy .Results:(1) Compared with baseline conditions , all the subjects showed clinical improve-ments in all the parameters evaluated 2 months post therapy , P<0 .05 .( 2 ) Significant difference were observed in the average PD changes between Group A [(2.15 ±0.42) mm], Group B [(1.76 ±0.29) mm] and Group C [(1.57 ±0.33) mm], P<0.05.No significant difference was observed in the aver-age PD changes between Group B and Group C , P=0.354.Significant differences were observed in the average pPD changes between Group A [(2.45 ±0.43)mm] and Group C[(1.90 ±0.48) mm], P<0.05.No significant difference was observed in BI and BOP% changes between Group A ,Group B and Group C.Conclusion: For patients with severe chronic periodontitis , it is safe and feasible to receive full-mouth SRP by 2 times within 1 week.The short-term ( 2 months ) advantages in PD changes are observed in patients receiving SRP and antibiotic usage at the same time comparing with patients using antibiotics after SRP or SRP alone .