Purpose: To comparatively evaluate the effect of a 5% boric acid (BA) irrigant on periodontal condition, bacterial level and oral neutrophil numbers with a 1% povidone iodine (PVP-I) irrigant as an adjunct to scaling and root planing (SRP) in chronic periodontitis (CP) treatment. Materials and Methods: A single-masked, randomised clinical trial with 36 CP patients was conducted at the Faculty of Odonto-Stomatology, University of Medicine and Pharmacy, Ho Chi Minh City, Vietnam. Subjects were randomly divided into two treatment groups: 1) SRP plus PVP-I 0.1% irrigant and 2) SRP plus BA 0.5% irrigant. Clinical measurements, including the plaque index (PI), gingival index (GI), bleeding on probing (BOP), probing depth (PD), clinical attachment level (CAL), bacterial level in subgingival plaque (BANA test) and the quantification of oral neutrophils were evaluated at baseline, 4, 6 and 8 weeks after treatment (T0, T4, T6 and T8). Results: Whole-mouth (PI, GI, BOP, PD, CAL and PD) parameters, bacterial level in subgingival plaque and number of oral neutrophils decreased statistically significantly after treatment compared to baseline in both groups (p 0.01). Between the two groups, whole-mouth PI, GI, BOP, PD and CAL reduction in the BA 0.5% group were higher than those in the PVP-I 0.1% group, but statistical significance was found only for GI and BOP after treatment (p 0.05). The PD and CAL reductions for moderately deep pockets (PD ≥ 5 mm and 7 mm) were significantly greater in group 2 compared to group 1 after treatment compared to baseline (p 0.01). This difference was not found for deep pockets (PD ≥ 7 mm). Conclusion: The results of this study suggest that BA 0.5% could be an alternative to PVP-I 0.1%, and might be more favourable because it provided superior results regarding whole-mouth BOP, GI as well as PD and CAL reduction for moderately deep pockets after CP treatment. Keywords: boric acid, chronic periodontitis treatment, povidone-iodine, subgingival irrigation