This in vitro study evaluated the compressive fracture resistance of marginal ridges of teeth with Class II tunnel preparations. Tunnel preparations were completed in extracted human bicuspid and molar teeth (bicuspids, bur size: #2, #4, #6; molars, bur size: #4, #6, #8). Control teeth in both the molar and bicuspid groups remained unprepared. Half the preparations in each group were restored with a glass ionomer cermet. After thermocycling, each marginal ridge was loaded in compression in an Instron Universal testing machine. Fracture values were compared with a two-factor ANOVA. A Dunnet post hoc test was used to compare the strength of each group to its control. Results indicated that up to a certain size, tunnel preparations could be completed without diminishing the strength of the undermined marginal ridges. Placement of a glass-ionomer cermet in the tunnel preparations restored the strength of all but the largest preparations (#6 bur in the bicuspid group) to a level which was not statistically significantly different from the controls.