A scenario study was conducted to assess the extent to which the unintended pregnancy rate in Japan, where oral contraceptives (OC) have not been legalized for family planning purposes and couples rely mainly on condoms , might change if more women were to use OC. Because current rates of unintended pregnancy and abortion in Japan are not known, data provided by the 1994 Japanese National Survey on Family Planning were used to construct scenarios for national contraceptive use. Annual failure rates of contraceptive methods and nonuse were applied to the contraceptive use scenarios, to obtain estimates of the annual number of contraceptive failure-related pregnancies. Subsequently, contraceptive practice situations assuming higher OC use rates were defined, and the associated change in the number of contraceptive failure-related pregnancies was estimated for each situation. It emerged that OC use rates of 15% decreased the expected number of unintended pregnancies by 13–17%, whereas use rates of 25% resulted in decreases of 22–29% and use rates of 50% in decreases of 45–58%. The findings were reasonably robust to variation in the assumptions that were made. In conclusion, each theoretical percentage increase in the OC use rate in Japan was found to lead to a roughly equivalent percentage decrease in the number of unintended pregnancies.