This in vitro study compared water sorption for various intervals over a 3-year period for visible light-cured microfill and hybrid composite resin with and without unfilled resin dilutions. Statistically significant sorption increases were evident for the 960-day interval for certain dilutions. At 3 years the microfill control revealed a 0.72% relative water sorption increase and its various dilutions 1.92% to 2.36%. By comparison, the hybrid control exhibited an increase of 0.4% and its dilutions were 0.72% to 0.98%.