Abstract Background Preoperative L-lactatemia and heart rate have been suggested as prognostic indicators of outcome for cows with right dilatation of the abomasum or volvulus (RDA/AV). However, postoperative L-lactatemia has not been assessed as a potential prognostic tool. Objectives To determine the prognostic value of postoperative L-lactatemia (LAC2), duration of treatment (Dt), relative L-lactatemia difference (compared with preoperative L-lactatemia [LAC1]) ([LAC2 − LAC1]/LAC2) and change in L-lactate over time ([LAC2 − LAC1]/Dt) as compared to preoperative findings (LAC1 and heart rate [HR]) as prognostic factors in dairy cows with RDA/AV. Animals A total of 41 dairy cows were included: 19 with AV and 22 with RDA; 11 cows had a negative outcome (NO) and 30 cows had a positive outcome (PO) based on telephone follow-up with owners 30 days after surgery. Methods Prospective cohort study. Analysis was performed using logistic regression and comparison of area under the receiver operating characteristics curve (AUC) using nonparametric tests. Results LAC1 > 1.4 mmol/L or LAC2 > 2.2 mmol/L had the same accuracy with sensitivity of 100% (95% CI, 75.1–100%) and specificity of 80% (95% CI, 61.4–92.3%) for predicting NO. The relative L-lactatemia difference ([LAC2 − LAC1]/LAC1) or lactate kinetics ([LAC2 − LAC1]/Dt) were not associated with prognosis. The AUC of the preoperative model (which included HR and lnLAC1) was 0.92 (95% CI, 0.83–1.0) and that of the postoperative model (including only lnLAC2) was 0.95 (95% CI, 0.88–1.0); these were not significantly different. Conclusions and Clinical Importance Postoperative L-lactatemia is helpful to predict outcome in cows with RDA/AV. The short-term change in blood L-lactate is not a useful prognostic indicator, at least during the period of time spent on the farm for surgery and treatment.