Sinusitis poses a difficult clinical challenge in the management of patients with AIDS because of high rates of relapse and the association with unusual pathogens. To determine the prevalence and severity of sinus disease in this group we prospectively analysed the condition of the paranasal sinuses shown on cranial MR scans of 156 patients referred for the investigation of suspected intracranial pathologies. These included 104 HIV seropositive patients, including 93 with an AIDS-defining diagnosis (CDC IV). Forty-two scans were performed on age-matched controls. The scans were timed to control for seasonal variations in sinus disease and were interpreted by two radiologists who were blinded to the clinical and serological status of the patients. Severe mucosal disease (more than one sinus showing >75% obliteration) or moderate mucosal disease (only one sinus showing >75% obliteration) was seen in 15.1% (14/93) patients with AIDS and none of the 42 controls (λ2=6.73, P<0.01). The mean maximum mucosal thickness in patients with AIDS was significantly greater than the control group (P<0.001) and also significantly greater than in seropositive patients who had not had an AIDS-defining diagnosis (CDC II/III) (P=0.006). Paranasal sinus mucosal abnormalities seen on MRI are greater in prevalence and severity in patients with AIDS and about one in seven would be expected to have at least one sinus largely obliterated. © 1993, All rights reserved.