Research on the effect of dietary ethanol by-products on beef quality has been limited. Some Universities have reported a decrease in marbling due to distillers grains inclusion, while others have not. It is unclear why marbling deposition may be decreased when increasing amounts of distillers grains are fed; however, decreased starch availability, increased vitamin A and D, and the high oil content in ethanol by-products may contribute. In contrast, distillers grains can increase unsaturated fatty acid content of beef, thus increasing healthfulness. Our objective was to measure the effect of wet distillers grains (0, 20, or 40 % of the diet) on growth, feed intake, and marbling deposition and to determine what may be responsible for decreased marbling. Average daily gain and feed intake did not differ between wet distillers grains treatments, but cattle fed distillers grains were more efficient. Marbling score decreased in high-concentrate-fed steers as WDG concentration was increased, but increased in high-foragefed steers from the 0 to 20% WDG inclusion rate and then decreased from the 20 to 40% WDG inclusion rate. Backfat thickness decreased in high-concentrate-fed steers as WDG concentration increased but increased in high-forage-fed steers from the 0 to 20% WDG inclusion rate and then decreased from the 20 to 40% WDG inclusion rate. Cattle fed distillers grains had lower plasma total vitamin A and plasma vitamin D. Retinol, however, was positively related to marbling and vitamin D was negatively related to marbling. Polyunsaturated fatty acids, which can enhance the healthfulness of beef, were increased by feeding wet distillers grains, but were related to decreased marbling.