The present study was aimed at comparing the milk urea nitrogen (MUN) and milk fatty acid (MFA) compositions in Holstein cows with subacute ruminal acidosis (SARA) to those values of Holstein cows that did not have SARA. Also, the correlations among rumen pH value and the compositions of MUN and MFA in milk were determined. Dairy cows (n = 16) with subacute ruminal acidosis (SARA) (pH value 5.60 +/- 0.02) and control dairy cows (n = 16) (control) (pH value 6.20 +/- 0.04) were studied. The MUN concentrations (578 mu g/l) of the dairy cows with SARA was lower than those (1 315 mu g/l) of the control dairy cows (P < 0.001). In the milk of the dairy cows with SARA, the unsaturated fatty acids (UFA), thrombogenic index (TI), and hypocholesterolemic fatty acid index (hcFA) decreased; but the saturated fatty acids (SFA), atherogenic (AI) and hypercholesterolemic fatty acid (HcFA) indexes (P < 0.01) increased. The rumen pH value and the concentration of the MUN were positively correlated with the proportions of the monounsaturated fatty acids (MUFA), polyunsaturated fatty acids (PUFA), omega-3 fatty acids (n-3), omega-6 fatty acids (n-6), omega-9 fatty acids (n-9), long-chain fatty acids (LCFA) and very-long-chain fatty acids (VLCFA) and the n-3/n-6 ratio of the milk samples (P < 0.05). Consequently, the dairy cow with SARA that are in early-lactation can affect the carbohydrate fermentation, fatty acid hydrogenation and protein degradation. The MUN concentration in the dairy cows with SARA seriously decreased. The SARA changes the milk fatty acid composition and decreases the MUFA, PUFA, n-3, oleic acid and hypocholesterolemic fatty acids and the hypocholesterolemic/hypercholesterolemic ratio (h/H) values of milk. Therefore, the nutritional and functional quality for human nutrition decreases in the milk of dairy cows with SARA.