Objective-To investigate the prevalence of radiologically-diagnosed paranasal sinus disease in HIV-1 seropositive patients. Subjects and setting-476 patients admitted to a dedicated inpatient unit for HIV and AIDS at the Middlesex Hospital, London, between September 1988 and February 1992. Design-Retrospective review of patients' case notes and radiological records. Results-30 patients (6.3%) had radiological evidence of paranasal sinus disease. At the time of admission, sinusitis was in the differential diagnosis in only 12 of the 30 patients; 13 patients were initially diagnosed as having meningitis. Pseudomonas aeruginosa was the causative organism in four patients, all of whom had advanced HIV disease. All four responded to appropriate antibiotics but had early relapse of infection. Conclusions-Sinusitis is an important and under-recognised cause of morbidity in patients with HIV disease.