Purpose: The aim of this study was to investigate implant survival and the reactions of the bone and mucosa of the maxillary sinuses in patients treated with fixed dental prostheses supported by multiple zygomatic implants. Materials and Methods: Between 2001 and 2007, patients with extensive resorption of the basal bone of the maxilla received multiple zygomatic implants without grafts. In 2012, the maxillary sinuses were examined with cone beam computed tomography. Neo-osteogenesis at the posterior wall of the sinus was considered to be a sign of osteitis and classified as mild (3 to 4 mm), moderate (4 to 5 mm), or severe (> 5 mm). The maxillary ostium was examined and accessory ostia were registered. The sinus mucosa was evaluated according to the Lund-Mackay staging system, and the sinus was checked for fluid level and atelectasis. Patients reported any nasal- and sinus-related symptoms. Results: Fourteen patients who had received 58 zygomatic implants and 13 standard implants were examined. The mean follow-up was 9.3 years, and the implant survival rates were 97% and 92% for zygomatic and standard implants, respectively. Osteitis was found in all but one patient and classified as severe in 17 of the 27 investigated sinuses. The infundibulum was obstructed in nine sinuses, and an accessory ostium was present in seven. The mucosa of the maxillary sinus showed signs of chronic rhinosinusitis in all but one patient. No fluid levels were observed. Atelectasis occurred in 15 sinuses. Twelve patients reported unaltered nasal and sinus status. Conclusion: Patients with extensive bone resorption in combination with voluminous maxillary sinuses and nasal cavities can experience long-term success with prostheses supported by multiple zygomatic implants. Asymptomatic chronic rhinosinusitis with osteitis and gradual collapse of the maxillary sinus cavity can be anticipated. Keywords: chronic rhinosinusitis, dental implant, osteitis, zygomatic implants