The prevalence and severity of periodontitis is considerably lower than was previously estimated. The available epidemiological methods are based on the premise that loss of periodontal attachment is a unique sign of periodontitis. However, additional factors resulting in loss of periodontal attachment confound data obtained from modern studies. The physiological process of continuous tooth eruption results in loss of periodontal attachment over a lifetime, at an approximate rate of 0.1 mm/year. Anthropological and clinical evidence for continuous tooth eruption is reviewed. The position of the alveolar crest appears to be stable; loss of periodontal attachment occurs as the teeth erupt and less of the root is embedded in bone. Retrograde periodontitis caused by pulpal disease is a known cause of severe, localised destruction of periodontal tissues. Its signs and symptoms include periodontal pocket formation, purulent inflammatory exudates, angular bone loss, swelling and bleeding of the gingival tissues and increased tooth mobility. Anthropological, experimental and clinical evidence for retrograde periodontitis is discussed. Misdiagnosis of angular (hemiseptal), furcal and other forms of localised periodontal damage as periodontitis has probably resulted because of the inability to determine the quality of pulpal health and the assumption of a gingivitis-periodontitis continuum. New epidemiological methods need to be developed in which physiological and all pathological conditions which affect the periodontal attachment are recognised and appropriately categorised.