We studied the frequency and timing of abortion and the serum levels of 17β-estradiol and progesterone in Korean native cows fed pine needles during pregnancy. Fifteen pregnant cows were randomly assigned to groups of 5. The control group was fed a concentrate and rice straw, and the other 2 groups were fed, in addition, either 1.3 or 2.7 kg (dry weight) of Korean pine needles daily, starting at an average of 91 d of gestation and continuing until 245 d of gestation. The health status of the dams and the viability of the fetuses were ascertained by rectal palpation and ultrasound scanning during pregnancy. Simultaneously, blood samples were collected for analysis of serum 17β-estradiol and progesterone. Two abortions in mid-pregnancy (at 126 and 150 d of gestation) occurred in the group ingesting the higher daily amount of pine needles. Premature parturition occurred at 259 d of gestation in this group and at 241 and 252 d of gestation in the group ingesting the lower daily amount of pine needles. The serum 17β-estradiol concentration was significantly higher (P < 0.05) at 4 mo of gestation and the serum progesterone level significantly lower (P < 0.05) at 8 mo of gestation in the group ingesting more pine needles daily than in either of the other 2 groups. These results suggest that the ingestion of pine needles may play a role in abortion in Korean native cows by increasing the serum 17β-estradiol concentration and decreasing the serum progesterone concentration.