The aim of this research was to determine the influence of Neospora caninum (NC) infection on reproductive parameters of Holstein dairy cows. The study was performed at 32 farms in the south of Brazil, latitude 27°30′S to 31°S and longitude 51°30′W to 55°30 W Farms included in this study met rigorous vaccination schedule for infectious bovine rhinotracheitis, bovine viral diarrhea virus, leptospirosis, and brucellosis. Blood samples collected from the dairy cows were submitted to serologic testing by an indirect fluorescent antibody test (IFAT) and were distributed into 2 groups: seropositive (GP; n = 160) and seronegative (GN; n = 558). The dogs (n = 51) of the farms were tested by IFAT. After abortion, fetuses were frozen at -80°C and submitted to PCR detection of NC in tissues. Seropositive cows that aborted were also tested by IFAT. Pregnancies of GP and GN cows were mon- itored by periodic ultrasound exams. After parturition or abortion all cows in GP and GN groups were submitted to gynecological exams. Cows that developed disease (clinical mastitis, metabolic disease, or laminitis) after parturition or abortion were excluded from the analysis to prevent any distortion of data. Farms were divided into 3 categories according to production technology, specifically, average milk production per cow per lactation: high (>25 kg), medium (15-25 kg), and low (< 15 kg). Data was analyzed by ANOVA using SAS^® software (SAS Institute, Cary, NC, USA) at a level of 5% significance and Student’s t-test for average reproductive parameters. The alternative hypothesis of the study was that at least 1 reproductive parameter was influenced in GP after abortion compared with GN cows that aborted. The null hypothesis was that there was no influence of NC antibody titer on reproductive parameters. The prevalence of antibody for NC in dairy cows was 24.48%. There was no prevalence difference among production systems (P > 0.05). The prevalence in dogs was 48.6% and did not differ among production systems. Fifty-nine (41.37%) NC-seropositive cows aborted. This differs significantly from the seronegative cow abortion rate (n = 17; 2.6%; P < 0.001). Occurrence of retained fetal membranes (RFM), endometritis, absence of uterine infection after abortion, and days between abortion and first estrus can be observed in Table 1. Abortion occurred between the fifth and eighth month of pregnancy. Occurrence of RFM was more frequent in cows that were seropositive. The period between abortion and first estrus was 79.06 v. 65.8 days for seropositive and seronegative groups, respectively, which differed significantly (P < 0.001). The results suggest that the higher occurrence of RFM after abortion in dairy NC-seropositive cows affects uterine involution, resulting in a delay of ovarian activity.