Analyze the variability of anatomic relations between the sigmoid sinus and other surgical landmarks within the mastoid cavity to establish a classification of sinus location to facilitate surgical planning.A descriptive study of surgical landmarks using 96 temporal bones from the Temporal Bone Laboratory at the Department of Otorhinolaryngology, Valle University at Cali, Colombia.A dissection simulating a canal wall-up mastoidectomy extended to include exposure of the facial recess, epitympanum, and sigmoid sinus was performed on 96 adult disease-free temporal bones. Seven standard measurements were taken on each bone between the sigmoid sinus and other structures in the mastoid cavity. All data were analyzed using Epi-Info 2000.This parametric study was consistent with three classes of anatomic variations in the location of the sigmoid sinus (type 1, type 2, and type 3) based on a Gaussian distribution of the seven variables.Variability in the anatomic location of the sigmoid sinus in the mastoid cavity is common and has surgical relevance.This classification will help to rapidly identify the sigmoid sinus relationship within the mastoid cavity and to use the sigmoid sinus as a landmark during ear surgery in those cases in which traditional landmarks, such as the posterior auditory canal wall or the semicircular canals, are destroyed by a pathologic process.