Our primary objective was to determine the relationships between Fasciola-specific antibody levels in bulk-tank milk and measures of productivity to estimate economic losses that are associated with Fasciola infections. A bulk-tank milk sample was collected in March 2004 from 1105 dairy herds in Flanders and the antibody levels against Fasciola hepatica (ODRf) and Ostertagia ostertagi (ODRo) were determined. The association of ODRf with four production parameters (milk yield, milk-protein %, milk-fat % and inter-calving interval) was assessed by multivariable linear-regression models. Production data were available for 463 out of the 1105 herds sampled. An increase in ODRf from the 25% quantile (0.428) to the 75% quantile (1.064) was associated with a decrease in the annual average milk yield of 0.7kg/(cowday) (P=0.002), with a decrease in the average milk-fat % of 0.06% (P<0.001) and with an increase of the mean inter-calving interval of 4.7 days (P=0.03). No significant relationship was found with the average milk-protein %. When the relationships of ODRf and ODRo with milk yield were tested simultaneously, we saw an additive rather than synergistic effect of concurrent infections.