OBJECTIVE Dental plaque has a harmful influence on periodontal tissue. When a porcelain restoration is fabricated and refinishing of the glazed surface is inevitable, the increase in surface roughness facilitates the adhesion of plaque and its components. The aim of this in vitro study was to evaluate the effect of surface roughness of glazed or polished porcelain on the adhesion of oral Streptococcus mutans. METHODS A total of 80 metal-ceramic specimens were prepared in the form of disks from two porcelain materials and divided into four groups according to the method of surface finishing. Surface roughness values (Ra-microm) for all specimens were recorded using a profilometer. S. mutans bacteria were isolated from saliva and all specimens were inoculated in test tubes containing a bacterial suspension allowing adhesion of the microorganisms to the specimens to occur. After incubation for 24 hours at 37 degrees C, the specimens were transferred to a sterile saline solution and an inoculum of 0.1 ml from each selected dilution was spread on the selective medium, mitis salivarius bacitracin agar (MSB). Bacterial counts, expressed in colony forming unit (CFU) taking into consideration the dilution factor, were recorded. RESULTS There was significant correlation (p<0.05) between surface roughness values (Ra-microm) and the amount of bacterial adhesion (CFU x 10(3)). The glazed surface was the smoothest and exhibited the least amount of bacterial adhesion. CONCLUSIONS A positive correlation between surface roughness and the amount of S. mutans adhesion was observed. The glazed porcelain surface was considered more biocompatible than other methods of porcelain surface finishing. CONCLUSIONS Chairside adjustments of the cervical contour or occlusal surface of porcelain restorations are sometimes necessary before or after cementation. Ideally, an uncemented restoration should be returned to the laboratory for reglazing after all adjustments have been completed. It is important to evaluate various polishing procedures used for these adjusted surfaces to achieve a finished surface that as closely as possible approximates the quality of glazed porcelain.