The purpose of this paper is to investigate the high rate of seroconversion for Neospora caninum in a dairy herd. Forty-five of 95 (47%) of the seronegative animals seroconverted within a period of 6 months. Seropositive animals were not equally distributed among age-groups. A high seroprevalence age-group of 8-30 months was housed together during a period of 4 months, indicating a point source infection within this period. A lack of association between the serological status of daughters and mothers also indicated horizontal transmission of the infection. Low avidity indices in the seroconverted animals confirmed a recent infection in this herd. However, there was no increased abortion rate in connection with the seroconversion, which indicates that mass seroconversions in dairy herds may remain unnoticed. There was circumstantial evidence that the farm dog played a role in the transmission of the infection to the cattle. However, also evidence was found for an unexplained ongoing horizontal transmission after the initial point source exposure.