['The aim of this study was to evaluate the quality of prediction for stable results after nonsurgical periodontal therapy by several microbiological variables of the subgingival biofilm and biomarkers of gingival crevicular fluid or oral lavage.', 'Forty-six individuals with moderate or severe chronic periodontitis receiving nonsurgical periodontal therapy were monitored for clinical variables, selected microorganisms, and biomarkers at baseline and 3 and 6\xa0months thereafter. Logistic regression analysis and general linear model (GLM) were applied for analysis of variance and covariance.', 'At 6\xa0months, 20 patients showed a high response (HR) to treatment (at least 60\xa0% of reduction of numbers of sites with PD >4\xa0mm), whereas 26 did not (low response, LR). All clinical variables were significantly improved at 3 and 6\xa0months within each group (p\xa0<\xa00.001, each compared with baseline). Modeling the impact of Porphyromonas gingivalis, Treponema denticola, and median of MMP-8 on to the response to treatment as continuous variables by GLM showed a significant influence of these variables (p\xa0=\xa00.045) with the strongest influence of P. gingivalis (p\xa0=\xa00.012) followed by T. denticola (p\xa0=\xa00.045) and no association with MMP-8 (p\xa0=\xa00.982). Samples tested positively for P. gingivalis decreased only in HR (3\xa0months: p\xa0=\xa00.003; 6\xa0months: p\xa0=\xa00.002). Calprotectin levels in GCF were lower in the HR group compared with the LR group at 3\xa0months (p\xa0=\xa00.008) and at 6\xa0months (p\xa0=\xa00.018).'] Is persistence of Porphyromonas gingivalis a negative predictor in patients with moderate to severe periodontitis after nonsurgical periodontal therapy?