In each of the two years, the 168 day feedlot period was divided into two equal periods of 77 days with an intervening 14 day adjustment period. During the first period, the animals in half of the pens within each breed-group received a high concentrate (85% grain) diet (H) while the rest of the pens received a diet (L) of hay ad libitum and half as much concentrate as the bulls on high concentrate. The L diet contained an average of 48% grain. In the second period, diets of bulls in half of the pens within each breed-group were switched from L to H and vice versa. During the feedlot period, the incidence of sickness among bulls was recorded. Disease incidence was higher in the second year compared to the first. In the second year, 49 out of 56 (87.5%) calves on the high concentrate diet required treatment for respiratory infections compared to only four (7.14%) in bulls on the low concentrate diet. The results suggest that a high concentrate diet for animals in feedlots may be more stressful to calves coming directly to the feedlot following weaning compared to a low concentrate diet, thereby acting as an additional predisposing factor to respiratory infections.