A dairy herd experienced an abortion epidemic during which 43 per cent of the cows at risk aborted. Neospora caninum infection was demonstrated in four of six fetuses suitable for examination and the group of at-risk cows that aborted had significantly higher N caninum antibody concentrations than the at-risk cows that delivered a live calf at term (P<0·001). The antibody concentrations in the cow herd were significantly higher than in the youngstock (P<0·001), and the concentrations in the youngstock increased significantly (P<0·001) with age. When seven months to a year old, the calves born at term to the at-risk cows had significantly higher (P=0·007) antibody concentrations than age-matched calves born before the epidemic. At the time of the epidemic, there was a significant increase in the antibody levels of the herd that was not consistent with vertical infection alone, indicating that there appeared to have been a sudden large increase in the incidence of horizontal postnatal transmission of N caninum to the cow herd, or to the surviving offspring of the at-risk cows, or to both of these groups.