OBJECTIVES: The objective of this study was to analyse MRI with morphological (T1, T2) and diffusion sequences (DWI, b1000) in patients presenting non-aggressive patterns of sinus opacity and operated on by functional endoscopic sinus surgery (FESS). DESIGN: A retrospective study. SETTING: Diffusion imaging in paranasal sinus pathology remains little known. To date, no imaging system is capable of determining the purulent content of a non-enhanced sinus filling. PARTICIPANTS: We included consecutive patients having undergone FESS in whom MRI of the paranasal sinuses was performed. Subjects were allocated to Case (pus) or Control (no pus) groups depending on sinus content found intraoperatively. FESS was performed for bacterial acute rhinosinusitis, acute exacerbations of chronic rhinosinusitis, non-purulent sinusitis, naso-sinusal polyposis, antrochoanal polyp, isolated polyp, angiomatous polyp and eosinophilic fungal sinusitis. Tumours, mucoceles and fungus balls were excluded. MAIN OUTCOME MEASURES: We analysed T1, T2, b1000 and MRI sequences and ADC map. RESULTS: On univariate analysis, intermediate signal in T2 and high signal in b1000 were associated with Cases (P < 0.001) as were low ADC values (P < 0.001). The difference in mean ADC values between Cases and Controls was statistically significant (respectively, 0.518 vs 2.041 × 10-3 mm2 /sec, P < 0.01). On multivariate analysis, MRI with ADC < 0.725 × 10-3 mm2 /sec and b1000_SI > brain was significantly associated with the case group. MRI with b1000_SI < brain and ADC > 1.450 × 10-3 mm2 /sec was significantly associated with the control group. CONCLUSIONS: Diffusion MRI offers extremely promising results regarding content characterisation of infectious sinus diseases.