To epidemiologically assess the periodontal conditions of the adult population in Switzerland. During a ten-year period, a total of 1318 subjects were randomly selected on the basis of a process involving the community rosters in seven regions (Cantons) of Switzerland: (Canton of Berne; Canton of Zurich; Western cantons (GE,VD,NE); Eastern cantons (SG,AI,AR,TG,SH); Southern cantons (VS,GR,TI); Central cantons (LU,GL,ZG,UR,SZ,OW,NW) and Northern cantons (JU,FR,SO, AG,BL)) and encompassing all adult age groups (20-89 years). The subjects were examined in dental offices randomly distributed throughout the country. The number of teeth present, the mean Plaque and Gingival as well as Retention Indices were assessed. Furthermore, pocket probing depths and loss of periodontal attachment were determined on all teeth. Ninety-four (7.1%) of the subjects were completely edentulous leaving 1224 dentate individuals with an average of 21.65 teeth for analysis. In the youngest cohort (20-29 years) 27.03 teeth were present. During the fourth to the seventh decade of life, tooth loss appeared to follow a linear pattern leaving 17.63 teeth in the age group of 60-69 years. The oldest age group of the 80-89 years old yielded 11.08 teeth. The mean scores of all clinical parameters increased significantly with increasing age. Mean PlI in the youngest age group was PlI: 0.72 (SD: 0.38) and reached PlI: 1.55 (SD: 0.68) in the oldest. The corresponding mean GI were 1.17 (SD: 0.31) and GI: 1.64 (SD: 0.50), respectively. The mean RI increased from 0.24 (SD: 0.29) to RI: 1.34 (SD: 0.70) in the oldest. Mean probing depth increased slightly from 20 to approximately 49 years. Thereafter, mean PPD remained around 3.0 mm. The loss of periodontal attachment increased dramatically after the age of 50. Periodontitis manifests itself after the age of 50. The pronounced progressive attachment loss in the age cohorts over fifty years explained the marked loss of teeth in some individuals. From a public health point of view, it is indicated to make a major effort for periodontal prevention in the population approaching the second third of life. Keywords: epidemiology, periodontology, prevalence, severity, Switzerland, periodontitis, gingival index, attachment loss