The liver functionality index (LFI) represents an assessment of periparturient cow metabolic health by measuring changes in 3 blood biomarkers (albumin, cholesterol and bilirubin). The aim of this study was to determine the effect of peripartal ethyl-cellulose rumen-protected Met supplementation on LFI groupings, and to assess relationships with animal performance and oxidative stress status during the periparturient period. Thirty multiparous Holstein were used in a block design and assigned to either a control diet or Met-supplemented (Mepron, Evonik Nutrition and Care GmbH, Germany) diet. Mepron was supplied from −28 to 30 d relative to parturition at a rate of 0.09% and 0.10% of DM during the prepartum and postpartum period, respectively. That rate ensured that the ratio of Lys to Met in the metabolizable protein was close to 2.8:1. Blood samples from 15 clinically healthy cows per treatment were collected at 7 and 30 d relative to parturition for biomarkers analysis. LFI was calculated using an updated equation [LFI = (Albumin - 17.71)/1.08 + (Cholesterol - 2.57)/0.43-(Bilirubin - 4.01)/1.21] and cows were ranked retrospectively into low LFI (LFI <0; n = 10) and high LFI (LFI >0; n = 20) groups regardless of Met supplementation. Most (67%) of the cows fell into the high LFI, which was composed mainly of Met-supplemented cows (P = 0.04; 13 Met vs 7 controls). Compared with cows in low LFI, postpartum DMI (P = 0.06; 17.7 vs 16.2 kg), milk yield (P = 0.05; 39.8 vs 34.8 kg), and ECM (P = 0.08; 47.1 vs 41.8 kg) were greater in high LFI cows. Furthermore, prepartum DMI (P = 0.12; 14.5 vs 13.3 kg) tended to be greater in high LFI compared with low LFI. The lower postpartum concentrations of reactive oxygen metabolites (P = 0.09; 17.7 vs 19.5 mg of H₂O/100 mL) and greater paraoxonase (P < 0.01; 97.6 vs 75.8 U/mL) and tocopherol (P = 0.03; 6.64 vs 5.25 mg/mL) in high LFI cows indicated a state of reduced oxidative stress. Β-carotene and retinol were not affected by LFI. Overall, feeding Met during the periparturient period results in higher LFI and better performance and oxidative stress status.