OBJECTIVE(S): To determine the accuracy of conventional and diffusion-weighted (DWI) MRI for the differential diagnosis between mucopyocele and mucocele using surgical diagnosis as a reference. METHODS: This retrospective study included patients referred to our institution between March 2017 and February 2024 for paranasal sinus MRI to characterize an expansile non-enhanced sinus filling on CT. The T1 and T2 signal intensities were recorded, as well as the presence of restriction diffusion, of the penumbra sign, the double rim sign, and the presence of parietal micro-abscesses. Quantitative apparent diffusion coefficient values were also recorded. RESULTS: The study included 74 patients aged 18 to 88 years (mean age: 60.4 ± 17.7 years). Of these, 43 had a mucopyocele and 31 had a mucocele. The T1 high signal intensity was different amongst groups as an hypersignal was present in 35.5% of mucoceles and only 4.7% of mucopyoceles (p = 0.02). The T2 signal did not differ between the two groups (p = 0.59). The double rim sign and the presence of parietal micro-abscesses demonstrated 96.8% and 100% specificities, respectively, but were predominantly located in the maxillary sinus. The penumbra sign exhibited a sensitivity of 60.5% and a specificity of 67.7%. All mucopyoceles exhibited a diffusion restriction with an apparent diffusion coefficient of less than 0.78 × 10-3 mm CONCLUSION: DWI is the most useful MRI tool for the accurate differential diagnosis between mucopyocele and mucocele in non-enhanced sinus fillings. LEVEL OF EVIDENCE: 3 (case-control study) Laryngoscope, 135:1008-1014, 2025.