Standardized class 4 cavities were prepared in bovine incisors and restored with a microfilled composite resin. The composite restorations were retained either by acid etching technique (AET) alone or by acid etching technique in combination with a self-threading retentive pin. Pins covered with a bonding/opaquer coating (PCR pin) and uncovered pins (FO pin) were used. After having been aged for 3 days, the fracture resistance of the restorations was determined with a Universal Testing Machine. The restorations were loaded at an angle of 45 degrees. The restorations retained by AET and a PCR pin showed the highest failure load. The restorations accomplished with AET and a FO pin yielded the lowest fracture load. The fracture toughness of the restorations retained by AET and a PCR pin was slightly but statistically significant, higher compared to the restorations exclusively attached by acid etching technique. It was concluded that there was only a small increase of fracture toughness of large class 4 composite restorations if the acid etching technique was combined with the application of bonding/opaquer-covered retentive pins.