Background and aims. Along with conventional periodontal therapy, subginigval application of platelet-rich plasma (PRP) may provide more effective improvements in clinical parameters due to the presence of multiple growth factors. The aim of this double-blind, split-mouth, randomized study was to evaluate the adjunctive use of PRP with scaling and root planing (SRP) in the treatment of chronic periodontitis. Materials and methods. A total of 87 non-smokers suffering from moderate to severe chronic periodontitis were selected. Parameters were probing pocket depth (PPD), clinical attachment level (CAL), plaque index (PI) and modified bleeding index (mBI). After full-mouth SRP the sites were randomly divided into experimental sites receiving subgingival application of autologous PRP and controls treated with placebo gel. Measurements were recorded at baseline, 3 months and 6 months. Paired t-test was used to compare response to treatment between the two sites. Results. Statistically significant changes in parameters were seen in both groups from baseline to 6 months. Inter-group comparison revealed significantly more clinical attachment gain for the experimental group (P>0.05). The mean CAL gain was 2.40±0.4 mm for control sites and 2.68±0.5 mm for experimental sites. Conclusion. This study supports the use of PRP during nonsurgical debridement of periodontal pockets measured 6 months after SRP.