Background and aims: A pneumocoele is a pathologically expanding, air-containing paranasal sinus. Pneumocoeles are uncommon entities, with very few cases reported in the literature. The most plausible aetiological theory is that of a one-way valve effect allowing air into the sinus under increased pressure without pressure equilibration.Methods: Review of a frontal sinus pneumocoele caused by a type III frontal cell and intersinus septal cell.Results: We present a patient with chronic sinusitis who developed a pneumocoele of her left frontal sinus with erosion into her orbit. This patient was a habitual, chronic nose-blower with a large type III frontal cell adjacent to an intersinus septal cell.Conclusion: We propose the new theory that, in our patient, such cells formed a one-way valve, causing air-trapping and pathological expansion of the frontal sinus. To our knowledge, there are no previously published cases that establish an association between large frontal cells and expansion of a pneumocoele. © 2009 Copyright JLO (1984) Limited.