Objectives. Evaluations of the marginal adaptation of restorations in clinical trials rely on subjective assessments by evaluators. The purpose of the present study was to assess the use of dental explorers with different tip diameters to detect simulated marginal discrepancies. Methods. Using three devices simulating vertical steps, horizontal gaps, and the combination of vertical and horizontal discrepancies and explorers with five different tip diameters, 10 experienced dental faculty members were asked to identify discrepancies at the boundary between Alfa (replacement unnecessarily) and Bravo (replacement questionable) ratings, according to modified United States Public Health Service criteria. Results. A significant correlation was found between the tip diameters of the explorers and the Alfa/Bravo boundaries for the horizontal gaps, but not for the vertical steps. Conclusion. It was concluded that the tip diameter of dental explorers had no significant effect on the detection of vertical steps, but had a significant effect on the detection of horizontal gaps. The diagnosis of restoration marginal discrepancies and the rating of marginal adaptation in clinical trials may subsequently be found to be best achieved using techniques other than the probing of restoration margins. © 2005 Elsevier Ltd. All rights reserved.