Objective To assess image quality and radiation dose associated with ultra-low dose CT protocol for patients with benign paranasal sinus diseases undergoing functional endoscopic surgery (FESS). Methods We scanned the head portion of Alderson RANDO phantom on a second generation, dual-source, multidetector-row CT scanner (Siemens Definition Flash) using standard-dose and five low-dose protocols. Two radiologists assessed the image quality for each protocol to determine best ultra-low-dose protocols for imaging patients with benign paranasal sinus diseases undergoing FESS. The ultra-low-dose CT protocols were then used for scanning. Thereafter, 40 adult patients (age range 18–54 years, M:F 23:17) were scanned with the four low dose scanning protocols (10 patients per protocol). On both transverse and coronal reformatted CT images, two radiologists assessed visibility of key anatomic landmarks for FESS on a 2-point scale (1 = clear and complete visualization ; 2 = suboptimal visualization). Data were analyzed with descriptive statistics and Cohen’s kappa coefficient for interobserver agreement. Results In phantom study, the lowest dose scan protocol (CTDI vol 2.1 mGy, 70 kV, 75 mAs) was unacceptable due to poor image quality. For patient studies, both radiologists gave acceptable image quality scores for ultra-low-dose scan protocol with axial scan mode, automatic tube potential selection and tube current modulation (CTDI vol 2.2 mGy ; DLP 22.9 mGy.cm) with up to 60% lower dose compared to prior standard-dose CT (CTDI vol 5.3 mGy ; DLP 73.5 mGy.cm). Conclusions Ultra-low-dose CT protocol provides sufficient image quality for scanning patients undergoing functional endoscopic surgery for benign paranasal sinus diseases.