The opacity of 5 self-curing and 11 light-curing composite resins was measured 24 hours after sample/preparation and after 6 months of storage (dry or in water) by means of an UV-visible spectrophotometer. The opacity of the investigated hybrid composite resins was between 47.3 and 62.4%. The opacity of the microfilled composite resins ranged from 48 to 64.4%. The opacity was not influenced by the polymerization time. The spectral opacity decreased with greater wavelengths, particularly in microfilled composite resins. All materials except Durafill, which became more translucent, were more opaque after 180 days of storage in water. Storage in water for 180 days caused greater changes of the opacity than dry storage for the same time. The opacity changes resulting from storage in water can be reduced by increasing the polymerization time.