This paper evaluates different aspects of sinusitis in patients with a decreased immunological system, such as its prevalence and clinical evolution, its peculiar bacteriology and the altered response to treatment, and the prognosis, especially in patients with AIDS. There seems to be an increased prevalence of sinusitis in these patients, with a relationship between their immunological status and the severity and aggressiveness of the sinusitis. Bacteriological studies reveal the pressure of more aggressive species, such as P. aeruginosa, and specific sinusitis are more frequent, which may explain why the treatment with common antibiotics often remains uneffective. The simultaneous therapy of concomitant infections leads to a higher resistance towards common drugs. A standard treatment is therefore needed. The results of three studies, retrospective and prospective, on HIV-infected patients reveal a high incidence of acute sinusitis with aggressive bacteria.