Chronic periodontitis, a common disease of microbial origin, is the major cause of tooth loss in adult humans. The disease serves as a convenient experimental model for analysis of many aspects of chronic inflammation. A consideration of currently available data has permitted the formulation of a new concept of the pathogenesis of this disease. The gingival tissues respond within 2 to 4 days to a beginning accumlation of microbial plaque with a classic acute exudative vasculitis which we have termed the initial lesion. This response, which includes loss of perivascular collagen, is comparable to that elicited in most other tissues subjected to acute injury and may be a consequence of the elaboration and release of chemotactic and antigenic substances by microbial plaque. Within 4 to 10 days, the early lesion develops. It is characterized by a dense infiltrate of lymphocytes and other mononuclear cells, pathologic alteration of fibroblasts, and continuing loss of the connective tissue substance. The structural features of the early lesion are consistent with those expected in some form of cellular hypersensitivity, and a mechanism of this kind may be important in the pathogenesis. The early lesion is followed by the established lesion which develops within 2 to 3 weeks and is distinguished by a predominance of plasma cells in the absence of significant bone loss. The established lesion, which is extremely widespread in humans and in animals, may remain stable for years or decades, or it may become converted into a progressive destructive lesion. Factors causing this conversion are not understood. In the advanced lesion, plasma cells continue to predominate although loss of the alveolar bone and periodontal ligament, and disruption of the tissue architecture with fibrosis are also important characteristics. The initial, early, and established lesions are sequential stages in gingivitis and they, rather than the advanced lesion which is manifest clinically as periodontitis, make up the major portion of inflammatory gingival and periodontal disease in humans.