The majority of patients with human immunodeficiency virus (HIV) infection will develop acute sinusitis. This may be a single episode, or may be the beginning of a long course of recurrent sinusitis, of which the etiology is not yet well understood. A retrospective study of cultures from antral washings was conducted to determine the organisms that were more commonly isolated in patients with HIV infection and sinusitis. Forty-seven organisms were isolated from the sinus cultures of 41 HIV-positive patients. The most common organisms isolated were Streptococcus pneumoniae (19%), Streptococcus viridans (19%), and Pseudomonas aeruginosa (17%). Pseudomonas aeruginosa is an atypical cause of acute sinusitis in the general population but was found to be an important pathogen in our HIV-infected patients. Other atypical organisms were also isolated, including Listeria monocytogenes and Candida albicans. It is important to recognize that atypical organisms must be considered if an HIV-infected patient with sinusitis does not respond to initial antibiotic therapy. A discussion follows emphasizing the need for prompt diagnosis and treatment of sinusitis in HIV infection.