The purpose of the present work was to investigate a possible relationship between the composition of composite restorative resins and their wall-to-wall polymerization contraction. The investigated brands contained bis-gma diluted with varying amounts of one or two low-viscous monomers. The wall-to-wall contraction was measured microscopically as the maximum gap width in the dentin part of cavities in extracted human teeth. A positive correlation between gap width and amount of diluting monomer was found, with commercial composites and with non-composite, experimental resins. Addition of up to 75°by weight of an inorganic filler to a monomer mixture similar to those found in commercial composites had no effect on the width of the contraction gaps. With non-composite experimental resins the gaps were found to increase with the amount of catalyst.It was concluded that the composition of the organic phase is of primary importance for the size of the wall-to-wall contraction of composite restorative resins.