Scanning electron microscopic investigations were performed on the maxillary sinus mucosa of five healthy persons and seven patients with chronic maxillary sinusitis. The occurrence of previously undescribed hair-like filaments in one case of chronic maxillary sinusitis is presented. These abnormal filaments are much longer (about 50--60 micron) and thinner (about 0.08 micron) than normal kinocilia (about 0.2 micron thick and 5--7 micron long), thus exhibiting a hair-like appearance. As the microvilli (cytofila) in maxillary sinus mucosa have the same thickness (about 0.08 micron), these hair-like filaments could be regarded as abnormally long microvilli. The possible significance of these abnormal filaments is discussed.