This study was designed to gain information on the composition and ultrastructure of bacterial plaque on the fitting surface of maxillary dentures in edentulous patients (with or without denture stomatitis of the palatal mucosa). Denture plaque was examined from 12 patients with complete dentures of unknown age. Clinical inspection of the palatal mucosa revealed that 5 suffered from denture stomatitis, while 7 exhibited a clinically normal palatal mucosa. Smears taken from the palatal mucosa and the fitting surface of the denture revealed the presence of bacteria in 11 of the 12 patients, while yeast cells were present in only 5, all of whom suffered from denture stomatitis. After the dentures were stained with a plaque disclosing solution, a 3 x 3 mm piece of the denture plate exhibiting plaque was cut out and processed for electron microscopy. Examination of sections in the light and electron microscope revealed no systematic differences between those from patients with denture stomatitis as compared with those with a clinically healthy palatal mucosa. In 2 specimens the denture was covered only by a very thin layer of condensed material which resembled the pellicle forming on teeth. In 10 the denture was covered with a deposit of microorganisms in an intermicrobial matrix. In 9 of these cocci or short rods predominated, and most of them were Gram-positive. In one specimen the plaque consisted of yeast cells only. The surface of the dentures was relatively smooth although minor irregularities were encountered. The denture material was in no instance invaded by microorganisms.