Objective The aim of this study was to identify causative organisms of acute rhinosinusitis in immunosuppressed patients by a retrospective chart review. Methods Records were reviewed using International Classification of Disease, Ninth Edition codes for inpatient treatment of acute sinusitis. Patients were included only if they had formal sinus cultures obtained and were immunosuppressed, either carrying a diagnosis of a hematologic malignancy or receiving chemotherapy. Demographics, underlying malignancy, and culture results were recorded. Results Records of 74 patients with 104 cultures were obtained. There were 43 males and 31 females. The mean age was 51. The most common primary diagnoses were leukemia (65%) and lymphoma (23%). Sixty cultures resulted in either no growth or growth of usual respiratory flora. Of the 44 positive cultures, 5 were polymicrobial, resulting in 61 organisms isolated in total. Bacteria cultured were 73% Gram positive, whereas 27% were Gram negative. The most common Gram-positive organisms cultured were Staphylococcus species. Pseudomonas species were the predominant Gram-negative bacteria. Thirteen samples grew fungal organisms. Conclusions Medical management of sinusitis in an immunosuppressed patient seems to be adequate in most cases. Patients who fail to improve should undergo evaluation by an otolaryngologist to obtain cultures for directed antibiotic therapy. Infections by Staphylococcus species as well as quinolone- and cephalosporin-resistant Gram-negative organisms may be encountered, and clinicians should suspect their presence in patients with persistent disease and expand their antibiotic coverage appropriately.