Background & Objective : Breast cancer is the most common type of cancer among women worldwide and accounts for the majority of their deaths due to the cancer. Hence, diagnosing its risk factors is of great importance. In this study the association between abortion and breast cancer risk was investigated.

  Methods : In this Case- Control study in Mashhad, 105 patients with confirmed breast cancer were compared with 105 healthy women who were living in their neighborhood and were the same age. A questionnaire including demographic information as well as same questions regarding the factors associated with conception and emphasising abortion were used to collect the data, which then were analyzed using SPSS ver 9.

  Results : the findings indicated a relationship between breast cancer and history of abortion, compared to women with no history of abortion (OR=2.09 95% CI=1.18- 3.69). Also there was a significant relationship between the frequency of abortion and breast cancer (P=0.008 ). but odds ratio was not higher among women whose age at first abortion was less than 20 years. Also abortion before first full- term pregnancy increased breast cancer risk (OR= 5.97 95% CI=1.18-19.6). But the age at the time of abortion did not have any effect on the breast cancer risk.

  Conclusions : The findings suggested that the risk of breast cancer has a positive relationship with frequency of abortion and the risk is even higher if the abortion occurs before the first full-term pregnancy. Thus, health interventions seem to be necessary in this regard.