Formation of hemorrhagic anovulatory follicles (HAFs) was studied daily in pony mares, beginning before ovulation (controls, n = 7) or HAF formation (n = 7). Solitary HAFs were used that were not accompanied by an ovulation from another follicle during the late follicular phase. The day of ovulation and the day of detection of excessive specks floating in the antrum were designated Day 0 for the controls and HAF group, respectively. There were no significant differences between groups in concentrations of systemic progesterone, LH, or FSH before Day 3, but estradiol was higher (P < 0.05) in the HAF group than in the controls on Day -3. The incidence in the controls of discrete ultrasonographic follicle indicators of impending ovulation (decreased turgidity, loss of spherical shape, echoic specks in antrum, serration of granulosum, and an apical area) was greater (P < 0.007) for Day -1 (2.3 ± 0.6 indicators/mare) than for Day -2 (0.1 ± 0.1), showing a temporal relationship to impending ovulation. There was no difference between groups in preovulatory indicators on either day. These results, as well as similar diameter between groups on Day -1, indicated that HAF formation occurred in a follicle that did not have altered B-mode ultrasonographic structure before the day of expected ovulation. Furthermore, the follicle cells were viable, as indicated by luteinization of the wall of the HAF; there were no differences between groups in circulating progesterone concentrations during Days 0 to 17. The percentage of follicle circumference with color-Doppler signals indicating blood flow on Day - 1 was greater (P < 0.03) in the HAF group (90 ± 4%) than in the controls (69 ± 7%). Results indicated that elevated plasma estradiol a few days before expected ovulation and greater vascularity of the follicle on the day before expected ovulation were associated with the conversion of a viable follicle into an HAF.