Thisin vivostudy assessed the remaining coronal tooth structure in teeth prepared for complete and partial coverage restorations using 3D-scanning and a Tooth Restorability Index (TRI). The cuspal coverage preparation designs selected by 10 postgraduate dentists and 10 general dental practitioners were recorded in a questionnaire. Eighteen patients had molar root treatment completed at the Eastman Dental Hospital and were prescribed a coronal-radicular amalgam core and cast restoration. Each tooth was preparedin vivoby one operator for a cast restoration. Two clinical impressions were made to produce two dies: one of remaining coronal tooth structure before crown preparation and a second die of coronal tooth structurein vivoafter crown preparation. For teeth prepared for partial coveragein vivo(n= 13), a third die was preparedin vitrorepresenting remaining tooth structure after complete coverage preparation. The three dies were of tooth structure prior to core placement. All dies (n= 31) were scanned using a laser profilometer and the volume of remaining tooth structure calculated. Four observers scored 31 dies using the TRI. The percentage loss of coronal tooth volume following a complete instead of a partial coverage preparation varied from 3·29% to 45·23% and the mean TRI fell from 10·7 to 7·5 units. There was a strong correlation between mean TRI and scanned volume of tooth structure (P= 0·013). Over 50% of the dentists altered their initial choice of restoration design from complete to partial coverage. Complete coverage preparations removed more tooth structure than partial coverage.