In dairy cows, the bulk tank milk (BTM) Ostertagia antibody level (ELISA, results as optical density ratio, ODR) is often used as a criterion for anthelminthic treatment at housing, one single BTM sample being collected between October and December. However, this autumnal BTM ODR could vary depending on the date of sampling. The objective of the study was to quantify the autumnal variability in BTM Ostertagia ODRs, taking into account the variability related to the measurement method. Ostertagia antibody levels were measured 16 times in 6 BTM samples (O. ostertagi-Ab ELISA kit, SVANOVIR®) (4 days, 2 experiments per day, samples tested twice per experiment, same operator). The coefficient of variation (CV), calculated for each series of 16 ODRs, ranged from 0.08 to 0.10. In 125 French grazing dairy herds, BTM samples were collected fortnightly from the 1st of October until housing and analysed with the same ELISA kit: 3 to 7 autumnal ODRs per herd were obtained. The effect of the sampling date on the ODR was assessed (linear mixed model, herd as random effect) and the CV of each within-herd series was calculated. November and December ODRs were slightly lower than October ODRs : -0.03 and -0.07, respectively (p =0.0002). The maximal ODRs were obtained during October in 99/125 herds (79%). Within-herd CVs ranged from 0.02 to 0.38 (mean 0.10) and 44 herds (35%) showed a CV > 0.10, indicating that the autumnal variation in BTM ODR values could be higher than the variability related to the method of measurement. Those results confirm that the BTM Ostertagia antibody level can vary during the autumn and this variability can have consequences on the decision to treat. To minimize the risk to under-estimate the exposure to Ostertagia, the optimal month for sampling BTM should be October.