The aim of this in vitro study was to provide first quantitative data on the marginal adaptation and the required load to fracture of adhesive composite crowns with and without adhesive composite cores. Eighteen caries-free extracted human premolars were restored using fine hybrid composite crowns with margins located entirely in dentin. Six crowns were adhesively luted to dentin stubs, six to endodontically treated teeth with adhesive titanium posts and adhesive composite cores, and six to adhesive composite cores without posts. Another ten unprepared teeth served as a control. All restored teeth were subjected to long-term occlusal and thermal stresses. The marginal adaptation was evaluated in the SEM before and after loading. Load to fracture was recorded at the end of the stress. Before stressing, 72.2 to 85.0%, and after stressing, 51.9 to 66.2% of "continuous margin" were recorded at the dentin-luting composite interface. The best results after stressing were achieved with crowns luted to adhesive composite cores without titanium posts. At the luting composite-composite crown interface, 61.6 to 88.7% of "continuous margin" before and 57.7 to 75.5% after stressing were recorded. The required load to fracture the restored teeth ranged from 72.0 to 89.2% of the unrestored, unloaded control. Adhesive composite cores without titanium posts yielded the best results.