This study measured maxillary sinus volume, evaluated the location of the semilunar hiatus in correlation to the nasal floor, and the incidence, location, and height of antral septa and discusses their clinical implications. Maxillary sinus volume was quantified in 65 cadavers (130 sinuses) by water application through the semilunar hiatus and measuring the used amount. The location of the semilunar hiatus was identified as distance from the nasal floor. The septa were counted, evaluated, and the size measured from the antral floor. The medium maxillary sinus volume was 12.5 mL (range, 5–22 mL). The medium location of the semilunar hiatus was 25.6 mm above the nasal floor (range, 18–35 mm). Thirty-five septa were counted in 130 maxillary sinuses. This equals an incidence of 27%. The medium height of the septa was 5.4 mm (2.5–11 mm). The main location of the septa was the region of the first molar (29%), the second molar (23%), and the second premolar (23%). The height, location, and number of septa as well as the height of the semilunar hiatus and volume of the maxillary sinus have to be taken into consideration to correctly plan the procedure and amount of grafting material in maxillary sinus floor elevation operations. Anat Rec, 292:352–354, 2009. © 2009 Wiley-Liss, Inc.