Twenty-nine specimens of the extraosseous portion of the human endolymphatic sac (ES) were serially sectioned longitudinally. The length and width of the extraosseous ES were measured and the surface area was calculated. As the specimens included the sigmoid sinus (SS), the relationship between the ES and the SS was analyzed. The extraosseous ES varied considerably in size. The lumen either consisted of a single tube or was subdivided into several tubules. The distal part of the ES overlapped the SS in one third of the specimens. The results indicate that a minute extraosseous ES could explain the sometimes difficult task of localizing this structure at ES surgery. The great variability in size might perhaps also explain the varying results of this surgery.