THIS STUDY investigated the relationship between the degree of periodontal destruction present in advanced periodontal lesions and consistently associated infiltrates of lymphocytes and/or plasma cells. A total of 385 biopsy specimens were obtained from the depths of pockets during routine periodontal surgery. The selected lesions were in "established" or "advanced" stages and were classified according to pocket depth as Class I (4-6 mm) or Class II (7-10 mm). The infiltrates attached to the cementum, bone, or lining the pocket soft tissue wall were classified in five types according to the distribution of lymphocytes, plasmocytes, and neutrophils. Statistical evaluation of the findings demonstrates a strong, significant relationship (P less than 0.001) between the degree of destruction of supporting periodontal tissues (Class II) and the predominance of plasma cells in the infiltrate (Type a). A weaker significant correlation (P less than 0.02) was found between predominant lymphocytic populations (Type b) and Class I lesions. The assumption was made that local, specific, cellular infiltrates reflecting immunologic mechanisms are associated with the degree of periodontal destruction in humans.