Asymmetries of the maxillary sinus with partial or total radiographic opacity mislead to the diagnosis of chronic sinusitis. Using three radiographic standard projections, computerised planimetric analysis and cephalometric studies helped to explain radiological opacity in 26 patients. So called "mucosal thickening" may be due to varying development of the zygomatic recess. Total opacity was caused not only by unilateral volume difference exceeding 30%, but also by straightning of the first lateral line in basal views of small and moderate sized sinuses. Cephalometric analysis in maxillary sinus asymmetry revealed ipsilateral transversal underdevelopment of facial- and skullbone. Reduced pneumatisation of the ethmoid, spenoid, mastoid and frontal sinus in diminishing order was also ipsilateral. Using photostat analysis there was nevertheless no correlation between facial- and coexisting maxillary sinus asymmetry.