The effects of estrogen and progesterone on the growth of dysgerminoma of ovary heterotransplanted to ovariectomized athymic nude mice (OVX-NUs) were examined. OVX-NUs were divided into 6 groups and a subcutaneous injection of either 1 micrograms or 10 micrograms of estradiol (E1 or E10), 0.1 mg or 1 mg of progesterone (P0.1 or P1), or a combination treatment of E1 and P1 (E1 + P1), or sesame oil as a vehicle control (oil) was given every other day for 2 weeks. Tumor size was recorded every day and the volume doubling time (VDT) was calculated. Tumor bearing OVX-NUs were killed on day 21 and tumor tissue was excised for retransplantation, histological examination and cytosol receptor assay. E10 treatment accelerated tumor growth and shortened VDT to 25 hours compared to 44 hours for the control group. P1 treatment delayed the tumor growth (VDT: 71 hrs) and E1 + P1 treatment further inhibited it (VDT: 197 hrs). E1 or P0.1 did not have any effect on the tumor growth. The levels of both the estrogen receptor (ER) and progesterone receptor (PR) of the original tumor were 5 fmol/mg protein, respectively. Hormone treatment had no remarkable effect, except for a significant increase in PR after estrogen treatment and this result seems to be responsible for the enhanced inhibitory effect of E + P on tumor growth. These effects were also observed histologically. In conclusion, the dysgerminoma used in this study was sex steroid hormone dependent and P alone or in combination with E could be a choice for the treatment of such ovarian dysgerminoma.