The objective of this study was to record how the antibody levels change over time during pregnancy in dairy cows naturally infected with the protozoan parasite Neospora caninum, and relate this to the reproductive performance. Eighteen cows with antibodies to N. caninum were serum sampled monthly during their first pregnancy and 13 of them were also followed for a second pregnancy. In all, five pregnancies ended in abortion and two in stillbirth. Antibodies to N. caninum in serum were demonstrated by immune stimulating complex enzyme-linked immunosorbent assay (iscom ELISA). The N. caninum antibody titres remained well above the 1:100 cut-off limit for the test used during 2 years in all cows. In the non-aborting cows, mean values of antibody titres to N. caninum rose 1.5-2.5 dilution steps to reach a plateau 4-5 months before parturition, and thereafter decreased from 2 months before parturition. These changes were statistically significant (p < 0.001). The same pattern was seen in the aborting cows. The consistent pattern of rise in antibody titres observed during both pregnancies in all cows indicated a reactivation rather than a reinfection of the parasite at mid-gestation.