Histological characteristics of the normal dog periodontium and of spontaneous periodontitis were studied in a necropsy material of 12 dogs of various breeds and ages. The material was selected on the basis of radiographical findings. The histological features of radiographically normal and diseased periodontium were found to be similar to those in humans. Progressive periodontal disease was characterized by dense subepithelial inflammatory infiltrates consisting predominantly of plasma cells intermingled with lymphocytes, epithelial migration beyond the cemento‐enamel junction and advanced loss of alveolar bone. A striking feature of the periodontal lesion was the narrow vertical extension of the process. Spontaneous periodontitis in dogs was closely related to accumulations of plaque and calculu