As a result of increasing use of bone marrow transplantation and new cytotoxic chemotherapy, more patients have become susceptible to sinus disease caused by unusual organisms. Sinusitis caused by fungi and gram-negative bacteria can be difficult to treat, may lead to severe complications, and should be managed promptly in the bone marrow transplant patient. Here we present the results of 41 cultures of the paranasal sinuses obtained from 18 bone marrow transplant patients in whom sinusitis developed. The most common agents were gram-negative bacteria (56.7%), followed by gram-positive bacteria (26.7%) and fungi (16.6%). In 13 samples the cultures were negative. Nasal cultures were performed ipsilateral to the sinus drained in 28 cases. Concordance was obtained in only 5 (17.8%) samples. The antibiogram of the isolated agents from the maxillary sinuses in this series revealed that the most efficient antibiotics were those that covered gram-negative bacteria. Treatment was usually prolonged in these patients, and different antibiotics were necessary to clear infections from the sinuses. In conclusion, treating sinusitis in bone marrow transplant patients may be challenging. Considerations about the microbiology and antibiogram susceptibilities of this specific population should be kept in mind when dealing with such cases.