This study was carried out to determine whether ruminal fermentation and serum acute phase protein concentrations differed between steers with and without pathology at slaughter. Ruminally cannulated steers (n = 28) were housed with noncannulated steers (n = 250) and randomly assigned to one of eight pens. Steers were previously adapted to a diet containing 5% barley silage and 95% concentrate. Ruminal pH was measured continuously and ruminal fluid and blood samples were collected weekly. Steers were slaughtered and those with a liver score (A or A ) or a rumen score of 2 or 3 (ulceration or scarring) were classified as pathology (PATH) and compared with those with no pathology (NOPATH). Steers with PATH spent 182 min d⁻¹ more with pH < 5.8 (P = 0.05) and 64 min d⁻¹ more time with pH < 5.2 (P = 0.03) than NOPATH. The concentration of serum amyloid A was greater for steers with PATH than NOPATH (P = 0.02) and haptaglobin tended (P = 0.08) to be greater. Overall, results of the current study suggest that the duration of time that ruminal pH is <5.2 and a serum acute phase protein response are greater for steers with a case definition including ruminal and liver lesions.