In randomized parallel longitudinal studies conducted in Norway between 1969 and 1995, and Sri Lanka between 1970 and 1990 the initiation and progression of periodontal disease was investigated. The two groups showed geographical, racial, cultural, socioeconomic and educational differences, and they represented extremes as to perceived severity of periodontal disease, general health care delivery systems and to dental care. The Sri Lanka population had never been exposed to any program of oral professional or self care. The Norwegian population, on the other hand, was exposed to conventional care throughout life starting at age 3 years. All results presented in this thesis are based only on analyses of the data collected from this 26-year longitudinal investigation in a group of middle-class men Norwegian men aged between 16 and 59 years. 565 subjects were recruited in 1969 and 223 subjects completed the study in 1995. A total of 7 surveys were staged during the observation period. Gingivitis was present in all subjects. The severity of gingival inflammation varied little throughout life. Initial loss of attachment was seen already at 16 years of age. Mean individual loss of attachment increased steadily during the thirties and forties and reached a maximum of 2.44mm at age 59 years. Sites that never showed any inflammation sings experienced the least mean cumulative loss of attachment (1.86mm) followed by sites with slight inflammation signs (2.25mm) over the 26-year observation period. In sites that consistently bled on probing over 26 years, the mean loss of attachment was 3.23mm. Gingival sites that bled on probing yielded an odds ratio of 3.22 to lose attachment as compared to healthy sites over the 26 year observation period. Throughout the observation period, 15% of the subjects accounted for the loss of 126 teeth out of 13285 teeth, an extremely low tooth mortality rate throughout 60 years of life. When teeth lost were analyzed relative to the long term status of their surrounding gingiva, it appeared that teeth surrounded by inflammation-free gingiva were maintained for a tooth age of 51 years, while teeth consistently surrounded by inflamed gingiva had a 46 times higher risk of being lost.