This study determined which light source was best at photopolymerizing five representative brands of resin composite. The hypothesis was that there would be no difference in the hardness of the composites when irradiated by any of the lights. Six curing light/tip combinations were used to photopolymerize five resin composites. In accordance with the manufacturer's instructions, the PAC light was used for 3 s and the high intensity QTH light was used for 5 s. The other QTH and LED lights were used for 40 s. To represent the clinical environment, the samples were irradiated at a distance of 2 and 9 mm away from the tip of the light guide. The Knoop hardness was measured at the top and bottom of the composites after 15 min and again at 24 h. The hardness data were compared using a general linear model analysis with Sidak's adjustment for multiple comparisons with p 0.01 as the level of significance. The 6 curing light/tip combinations had different effects on the hardness of the 5 composites (p 0.01). The two LED lights could not cure the neutral shade of Pyramid Enamel in 40 s. As the distance increased from 2 to 9 mm, the decrease in hardness was not similar amongst the different light/tips and composite combinations (p 0.0012). The curing light/tip combination which delivered the greatest total energy produced the hardest specimens. 1) The 6 curing light/tip combinations had different effects on the hardness of the 5 composites (p 0.01). 2) Neither of the two LED lights used was able to adequately polymerize the five resin composites tested. 3) The QTH light, which delivered the greatest total energy, always produced the hardest resin composite. 4) When the distance of the composites from the light guides was increased, the effect on their hardness was not the same for all light/tip combinations. It is therefore not possible to predict the performance of a curing light at 9 mm based upon power density measurements or hardness data recorded when the tip of the light guide is 2 mm away.