Holstein cows (n=221) from eight commercial dairy herds were examined for endometritis between 28 and 41 days postpartum using 5 diagnostic techniques: (1) vaginoscopy; (2) ultrasonographic assessment of uterine fluid volume; (3) ultrasonographic assessment of endometrial thickness; (4) endometrial cytology collected by cytobrush; and (5) endometrial cytology collected by uterine lavage. Concordance correlation was used to evaluate the reliability of cytobrush and lavage cytology. Cytobrush cytology was found to have the greatest intraobserver repeatability (cytobrush, rho(c)=0.85 versus lavage, rho(c)=0.76) and was chosen as the reference diagnostic test. Pregnancy data at 150 days postpartum was available for 189 cows. Survival analysis was used to determine the lowest percentage of polymorphonuclear cells associated with time to pregnancy. The sensitivity and specificity of the diagnostic techniques was determined using pregnancy status at 150 days and cytobrush cytology as the diagnostic standards. The risk of non-pregnancy at 150 days was 1.9 times higher in cows with more than 8% PMNs identified using cytobrush cytology than in cows with less than 8% PMNs (P=0.04). Twenty-one cows of 189 cows (11.1%) had >8% PMNs and were considered to be positive for endometritis. Cows with endometritis had a 17.9% lower first service conception rate (P=0.03) and a 24-day increase in median days open (P=0.04). The sensitivities of all five diagnostic tests relative to 150-day pregnancy status ranged from 7.1 to 14.3% and the specificities from 84.0 to 93.3%. Relative to cytobrush cytology, the respective sensitivity and specificity values are as follows: vaginoscopy (53.9%, 95.4%); lavage cytology (92.3%, 93.9%); ultrasonographic assessment of uterine fluid (30.8%, 92.8%); and ultrasonographic assessment of endometrial thickness (3.9%, 89.2%). Endometritis impaired reproductive performance. Cytobrush cytology was the most reliable method of diagnosing endometritis in cattle.