Purpose: The influence of tin plating on marginal seating and retention of complete cast crowns was evaluated. Materials and Methods: Thirty-six standardized gold-alloy complete crowns were made on extracted human molar teeth prepared with a modified milling machine using conventional laboratory techniques. The crowns were randomly assigned to two groups according to the treatment of the fitting surface (air abraded with 50-um aluminum oxide for 10 seconds, or tin plated with a microtin system for 3 seconds), and luted with adhesive resin cement (Panavia-EX, J. Morita). Scanning electron microscopy was used to evaluate the variables for the cast alloy. Retention was measured with an Instron testing machine after thermocycling (1,500 cycles between 5 degrees C and 55 degrees C with 1 minute dwell times). Marginal adaptation was measured with a Nikon microscope before and after cementation. Results were subjected to one-way analysis of variance and Student's t test. Results: Retention was significantly different (P 0.001); the highest retention (367 N) was reported in tin-plated crowns. Differences for marginal seating were not statistically significant. Conclusion: Tin plating after air abrasion was found to be a predictable and simple technique to roughen the casting surface, increase the surface area for bonding, and significantly improve the retention of cast crowns in vivo.