The clinical yellowing of anterior composite resin restorations with time is often the cause for their replacement. This work examined in vitro the effect of sunlight and water separately and together on color changes in the lighter shades of 16 resins. The natural radiation (ultraviolet and visible) was greater than would be expected in clinical use, and the color changes were generally only slight for most resins, as judged by standardized photographic recording and visual inspection of transparencies over time. The changes were greater when sunlight and water were combined. The light-cured surface was less affected than the distal surface. The hydrolysis action by water was particularly marked for one resin, which also showed a high release of camphoroquinone-type catalyst. Marked clinical discoloration may be as much or more affected by absorption of food colorants than by sunlight and water.