Marginal adaptation to cylindrical dentin preparations of five commercially available dental composites--Charisma, Estelite, Herculite, Pertac, and Silux Plus--and seven experimental dental composites containing prepolymerized filler was evaluated by measurement of the wall-to-wall polymerization contraction gaps. The experimental composites were prepared with varying amounts of spherical inorganic filler particles, irregularly shaped prepolymerized filler particles, and a base resin matrix composed of BIS-GMA and TEGDMA. Cylindrical dentin preparations, approximately 3.0 mm in diameter and 1.5 mm deep, were prepared in the exposed approximal dentin surfaces of extracted human molars. These cavity preparations were pretreated with an experimental dentin bonding system consisting of 0.5 mol/L EDTA, 35% glyceryl methacrylate, and Clearfil Photo Bond, then restored with each composite. Complete marginal adaptation was observed in two composites (Silux Plux and Estelite). Wall-to-wall contraction gaps were significantly related to the amount of ashed inorganic component in the composites (0.01 < P < 0.05) and to the spherical inorganic filler content by volume (P < 0.01).