Age standardized incidence rates of breast cancer in developed countries is nearly threefold higher than in developing countries. Iran has had one of the lowest incidence rates for breast cancer in the world, but during the last four decades increasing incidence rates of breast cancer made it the most prevalent cancer in Iranian women. After adjustment for age, Iranian young women are at relatively higher risk of breast cancer than their counterparts in developed countries. The purpose of this study was to investigate some established risk factors of breast cancer in Iranian young women. A hospital-based case control study comprising 521 women with histologically confirmed, incident breast cancer and 521 controls frequency-matched by age and province of residence was conducted. Logistic regression performed to investigate associations of reproductive and anthropometric factors with breast cancer risk. In multivariate analysis, family history [odds ratio (OR): 1.61; 95% confidence interval (CI): 1.07–2.42], oral contraceptives (OC) usage (OR: 1.52; 95% CI: 1.11–2.08), low parity (OR parity ≥ 3 vs. 1–2: 0.33; 95% CI: 0.23–0.49), employment (OR: 1.83; 95% CI: 1.05–3.23) and shorter period of breast feeding (OR ≥ 37 months vs. < 37: 0.61; 95% CI: 0.44–0.84) were related to a higher risk of breast cancer in young women. This was the first study focusing on risk factors of breast cancer in Iranian young women. The trend of decreasing parity and shortened duration of breast feeding along with OC usage might partly explain the rapid rising of breast cancer incidence in Iranian young women.