In attempt to identify major clinical features of paranasal sinusitis following allogeneic BMT, we reviewed 44 consecutive cases diagnosed at the Hammersmith Hospital between August 1993 and December 1995. All patients had symptoms and signs characteristic of sinusitis. Plain radiographs and/or CT scans revealed fluid levels in 86.4% of patients, opacification in 9.1%, and marked mucosal thickening in 4.5%. Two-thirds of patients were diagnosed within 120 days of BMT. The WBC was less than 1 x 10(9)/1 in 16.3% of patients, the neutrophil count was less than 0.5 x 10(9)/1 in 18.6%, and serum immunoglobulins were depressed (< 6.7 g/l) in 40.6%. Grade III-IV acute GVHD was present in 25.6% of patients and grade I-II in 66.7%; 68.6% developed chronic GVHD. There were 70.5% of patients receiving corticosteroids. Specific pathogens could not be identified in most cases. Pneumonia was present in 10 patients, seven of whom had Aspergillus species identified by bronchoalveolar lavage. Parainfluenza virus was isolated in three patients and Pseudomonas aeruginosa in two. Although all patients received antimicrobial therapy, 70.5% developed chronic sinusitis. Fatal complications did not occur. In 94 consecutive patients receiving allografts for CML during the period of study, WBC and neutrophil counts were lower 3 months post-BMT in patients who developed sinusitis (P < 0.02). Patients receiving higher doses of total body irradiation (13.2 and 14.4 Gy) had a greater probability of developing sinusitis (P = 0.023). Sinusitis occurred in only one of 37 patients receiving autologous transplants in the same period. Sinusitis is common following allogeneic BMT. Leukopenia is often present, but microbiological diagnosis is difficult, and progression to chronic sinusitis common.