The objective was to characterize ovarian follicular dynamics in beef cows treated with a CIDR (Bioniche Animal Health; Belleville, Ontario, Canada) and an injection of estradiol-17β (E2), with or without progesterone (P4), late in the estrous cycle. Previously synchronized, non-lactating, crossbred beef cows (n = 36) received a CIDR (Day 0) 16 to 18 days after ovulation and were randomly allocated to one of three treatment groups: no further treatment (Control, n = 12), an injection of 5 mg E2 (E2, n = 12), or 5 mg E2 plus 100 mg P4 (E2P4, n = 12; both from Sigma Chemical Co., St. Louis, MO, USA) i.m. in 2 mL canola oil. On Day 7, CIDR were removed and cows received 500 μg i.m. of cloprostenol (Estrumate, Schering Plough Animal Health, Pointe-Claire, Quebec, Canada). Ovaries were examined once daily by transrectal ultrasonography to detect ovarian follicle growth profiles, and determine the time of ovulation. Blood samples were taken daily for progesterone determination. Data were analyzed by ANOVA (LSD and Bartlett’s tests), Student’s t-test and chi-square procedures. Diameter of the CL and the dominant follicle, and progesterone concentration on Day 0 did not differ among groups (P = 0.6; overall mean (±SD), 16.8 ± 2.7 mm, 14.1 ± 2.0 mm, and 1.5 ± 1.9 ng mL, respectively). Thirteen cows ovulated within 3 days of treatment (50% of E2- and E2P4-treated cows and 8.3% of Control cows; P = 0.05); cows that ovulated had smaller CL diameters (15.2 ± 1.7 v. 17.7 ± 2.7 mm; P < 0.004) and lower progesterone concentrations (0.4 ± 0.2 v.2.1 ± 2.2 ng mL; P < 0.001) at the time of treatment. Follicular wave emergence occurred within 7 days in 4/12 Control cows, 10/12 E2-treated cows, and 10/12 E2P4-treated cows (P < 0.01). Although the interval from treatment to wave emergence did not differ among treatments (P = 0.8; overall, 3.4 ± 1.5 days), follicular wave emergence was more synchronous (P < 0.004) in the E2 group than in the Control or E2P4 groups. At CIDR removal, dominant follicle diameter was larger (P < 0.02) in the Control group (15.9 ± 5.5 mm) than in the E2 (11.9 ± 1.8 mm) or E2P4 (11.5 ± 3.4 mm)groups, but dominant follicle diameter was less variable (P < 0.003) in the E2 group than in the other two groups. Three cows did not ovulate after CIDR removal; two in the Control group and one in the E2P4 group. Interval to ovulation was shorter (P < 0.05) in the Control group (70.8 ± 10.5 h)than in the E2 (87.0 ± 9.0 h) or E2P4 (86.2 ± 7.2 h) groups, and the intervals to ovulation in cows that ovulated following treatment (91.0 ± 8.0 h) was longer (P < 0.001) than in those that did not (76.6 ± 9.6 h). In summary, treatment of cows with an estradiol-progesterone protocol late in the estrous cycle resulted in ovulation (50.0%), atresia (33.3%) or persistence (16.6%) of the dominant follicle present at that time. As length of follicular dominance and timing of ovulation were affected, fertility may be impaired following AI.