Purpose: The aim of this in vitro study was to investigate the retention characteristics of attachments fabricated by machine milling or by custom casting. In addition, the retention of reduced dimension attachments was also evaluated. Materials and Methods: Three types of ERA matrices, one prefabricated and two cast, were used. Ten specimens were made for each type, and white nylon patrices were transferred to denture bases. Tests were performed at a crosshead speed of 0.2 in/min with an Instron machine. The dislodging force at baseline, 100, 200, 300, 400, 500, and subsequently after every 500 pulls up to 2500 pulls were measured. ANOVA and Student's t-test were used to analyze the measurements. Results: All three groups showed no difference at the baseline, but the cast groups showed greater variation within group. The prefabricated group showed higher retention after 200 pulls. Decreased-dimension attachments have no significant difference when compared to the cast regular dimension group after 400 pulls. Conclusion: The prefabricated attachments had superior retention than the cast groups over time. Reduced dimension did not reduce the retention when compared to the cast group. © 2009 by The American College of Prosthodontists.