ObjectivesPeriodontitis disproportionately affects different racial and ethnic populations. We have previously reported the higher levels ofPorphyromonas gingivalisand lower ratios ofStreptococcus cristatustoP. gingivalismay contribute to periodontal health disparities. This prospective cohort study was designed to investigate if ethnic/racial groups responded differently to non-surgical periodontal treatment and if the treatment outcomes correlated to the bacterial distribution in patients with periodontitis before treatment.MethodsThis prospective cohort pilot study was carried out in an academic setting, at the School of Dentistry, University of Texas Health Science Center at Houston. Dental plaque was collected from a total of 75 African Americans, Caucasians and Hispanics periodontitis patients in a 3-year period. Quantitation ofP. gingivalisandS. cristatuswas carried out using qPCR. Clinical parameters including probing depths and clinical attachment levels were determined before and after nonsurgical treatment. Data were analyzed using one-way ANOVA, the Kruskal–Wallis test, the paired samplest-test and the chi-square test.ResultsThe gains in clinical attachment levels after treatment significantly differed amongst the 3 groups–Caucasians responded most favorably, followed by African-Americans, lastly Hispanics, while numbers ofP. gingivaliswere highest in Hispanics, followed by African-Americans, and lowest in Caucasians (p= 0.015). However, no statistical differences were found in the numbers ofS. cristatusamongst the 3 groups.ConclusionDifferential response to nonsurgical periodontal treatment and distribution ofP. gingivalisare present in different ethnic/racial groups with periodontitis.