Amat, S., McKinnon, J. J., Simko, E. and Hendrick, S. 2014. Evaluation of feeding corn or wheat dried distillers' grains with solubles on animal health of finishing feedlot steers. Can. J. Anim. Sci. 94: 525-531. Using dried distillers' grains with solubles (DDGS) as a replacement for barley grain is believed to reduce the incidence of rumen acidosis and liver abscessation due to its low starch content. However, feeding DDGS has the potential to induce polioencephalomalacia (PEM) owing to its high sulfur (S) content. The present commercial feedlot study was conducted to evaluate the effects of feeding corn or wheat DDGS on the health of finishing steers with a focus on the impact of these by-products on mortality, prevalence of ruminitis, cystitis, PEM and liver abscessation. The trial was conducted as a complete randomized design with three treatments using a total of 6815 crossbred steers in 10 pens per treatment. The control group diet consisted of 93.1% dry-rolled barley, 5.0% barley silage and 1.9% supplement (dry matter basis). The two treatment group diets included the replacement of 22.5% barley with corn or wheat DDGS. A complete necropsy and post-mortem were performed on all animals that died or were euthanized. In addition, rumen, bladder and brain tissues were collected for histopathological examinations. Liver abscess scoring was also performed. A total of 178 animals died and the overall mortality rate was not different between treatment groups (P=0.34). Bovine respiratory disease, lameness, metabolic disease, suspected Histophilus somni or miscellaneous mortalities did not differ (P>0.05) among the treatment groups. Incidence of ruminitis confirmed by histopathology in cattle either fed corn or wheat DDGS did not differ (P=0.16) from the control cattle. No particular bladder lesions, central nerve system infections or incidences of clinical PEM associated with feeding DDGS were detected. The prevalence of liver abscess at slaughter was not affected by treatment (P>0.05). The results of the present study indicate that feeding 22.5% corn or wheat DDGS in barley based finishing diet did not affect animal health of finishing feedlot steers.