To study the effects of bone involvement in experimentally induced sinusitis and the effect of involved bone on the overlying mucosa. Animal study. Sinusitis was induced unilaterally with Pseudomonas aeruginosa in the maxillary sinus of 19 New Zealand white rabbits. At 6 weeks, the pathogenic organism was confirmed by culture, and a segment of the bone from the medial wall of the sinus implanted in a submucosal pocket in the opposite sinus. The rabbits were killed at predetermined time intervals up to 13 weeks from sinusitis induction, and en bloc sinus sections were decalcified and stained. The implanted bone reabsorbed partially or totally in all specimens. However, the study revealed clear histological evidence of bone involvement adjacent to the infected sinuses and the bony changes extended to the noninfected side in all specimens. The histological findings were identical to those seen in chronic osteomyelitis. This study demonstrates the ability for pseudomonal sinusitis, at least in the presence of surgical intervention, to involve bone at a distance from the site of primary infection in the absence of intervening mucosal disease. If confirmed with additional organisms and models, these findings have significant implications for the therapeutic management of chronic sinus disease. Key Words: Rhinosinusitis, bacterial, osteitis, osteomyelitis, pathogenesis.