Statement of problem. It has been recommended that the anterior tooth midline be placed coincidently with the midline of the face (facial midline). The location of the facial midline depends largely on the judgment of the clinician. The amount of deviation between these 2 midlines that is noticeable to the observer has not been fully investigated. Purpose. The observable deviation between the anterior tooth and facial midlines in a limited sample of dentate subjects was recorded. Material and methods. Full facial-view, standardized photographs (x1/1.5) of 45 subjects, meeting very limited inclusion/exclusion criteria and undergoing routine dental treatment at Tel Aviv Dental School, were examined by 10 observers: 5 dentists and 5 nondental personnel who were given only a brief explanation of facial and tooth midlines. The examiners asked whether the facial and anterior tooth midlines deviated. The photographs then were scanned onto a computer screen, and the facial midline was determined by bisecting the distance between the medial angles of the eyes. The distance between a line perpendicular to this point and the contact point of the central incisors was measured by one calibrated examiner. The photographs were grouped according to the midline deviation: group 1, <1 mm; group 2, 1 to 2 mm; and group 3, >2 mm. Two photographs with oblique anterior tooth midlines were removed from the study because they were so easily detected. The observers' detection rates for the remaining midline deviation were compared and subjected to 1-way analysis of variance to identify significant differences at the 95% level of confidence. A post hoc Student t test was performed to identify significant differences among the groups. Results. Dentists and nondental personnel demonstrated a similar ability to notice deviations of anterior tooth and facial midlines. Midline deviations of <1 mm (group 1) were detected by 6 of 10 observers in 4 of 29 photographs (14%). Midline deviations of 1 to 2 mm (group 2) were detected by 6 observers in 3 of 8 photographs (37%). Midline deviations of >2 mm (group 3) were detected by 6 observers in 5 of 6 photographs (83%). The greater the deviation, the higher the detection rate. Significant differences were found between group 3 and the other 2 groups (P<.01). Conclusion. Within the limitations of this study, the greater the deviation of anterior tooth and facial midlines, the higher the detection rate. Nearly half of the 10 observers involved in this investigation were unable to detect midline deviations of <2 mm.