AIM:: The aim of this study was to investigate the association of serum vitamin B12 with the progression of periodontitis and risk of tooth loss in a prospective cohort study. MATERIALS AND METHODS:: In the Study of Health in Pomerania, 1648 participants were followed from 2002–2006 to 2008–2012 (mean duration 5.9 years). Serum vitamin B12 was measured by chemiluminescent enzyme immunoassay. Probing pocket depth (PD) and clinical attachment loss (CAL) were measured to reflect periodontal status on a half-mouth basis at each survey cycle. Tooth numbers are based upon a full-mouth tooth count. RESULTS AND CONCLUSIONS:: In multivariate regression models, baseline vitamin B12 was inversely associated with changes in mean PD (Ptrend = 0.06) and mean CAL (Ptrend = 0.01), and risk ratios of tooth loss (TL; Ptrend = 0.006) over time. Compared to participants in the highest vitamin B12 quartile, those in the lowest quartile had 0.10 mm (95%CI: 0.03, 0.17; Pdifference = 0.007) greater increase in mean PD, 0.23 mm (95%CI: 0.09, 0.36; Pdifference = 0.001) greater increase in mean CAL and a relative risk of 1.57 (95%CI: 1.22, 2.03; Pdifference < 0.001) for TL. Stratified analyses showed stronger associations between vitamin B12 and changes in mean CAL among never smokers (Pinteraction = 0.058). Further studies are needed to understand the potential mechanisms of these findings.