The aim of the present investigation was to clinically determine the thickness of masticatory mucosa in the hard palate and tuberosity as potential donor sites for ridge augmentation procedures. In 31 periodontally healthy, fully dentate subjects the masticatory mucosa thickness was assessed by bone sounding with a periodontal probe. Eighteen standard measurement points were defined in the hard palate, located on 3 lines which ran at different distances parallel to the gingival margin. Six positions were designated on each of these 3 lines between the level of the canine and the second molar. In the tuberosity, 6 standard measurement points were defined, located on 2 lines running parallel to the gingival margin at 2 different distances from the distal aspect of the second molar. Three positions were designated on each line. The hard palate and tuberosity were anesthetized by a spray followed by carticain injection with an epinephrine vasoconstrictor of 1:100,000. Data were analyzed to determine differences in gender, between different positions, and between lines, using an analysis of variance and Wilcoxon test. The mucosa of the tuberosity was significantly thicker than in the hard palate region. Gender did not influence the thickness of masticatory mucosa, either in the hard palate or the tuberosity with the exception of the most distant line in the palate. The mucosa was thickest at the mid-distal position of the tuberosity. In the hard palate, mucosa thickness increased with greater distances from the marginal gingiva. The mucosa over the palatal root of the maxillary first molar was significantly thinner than at all other positions in the hard palate. This represents an anatomical barrier in graft harvesting. It was concluded that two different regions may be defined for soft tissue graft harvesting from an anatomic point of view: 1) In the canine-premolar region rather wide and shallow grafts may be harvested. This region extends distally to the first palatal molar root with a significantly thinner mucosa. 2) The tuberosity revealed a significantly more soft tissue thickness in comparison to the hard palate. This region allows the harvesting of deeper grafts, but graft size is limited by the width of keratinized tissue.