To try to solve the pathogenesis of severe attic retraction viewed from mastoid condition, we examined the residual soft tissue density (RSTD) in the mastoid by computed tomography (CT) in 85 patients (107 ears) with otitis media with effusion (OME) 3 months after tympanostomy tube insertion or later. The incidence of RSTD in the mastoid was significantly higher in OME of adults (52.6%) than in children (24.1%). Ears with severe attic retraction had RSTD significantly more frequently (80%) than those with no or mild attic retraction, and many of the mastoids with severe attic retraction were occupied totally by RSTD. The area of the mastoid (mastoid pneumatization) was significantly smaller, and CT density of the mastoid (sclerotic tendency) was significantly higher in ears with RSTD than in those without. RSTD after tympanostomy tube insertion in the mastoid indicating organic change of effusion was considered one of the important factors relating to the pathogenesis of severe attic retraction.