Aim and Background. There are many genes as well as environmental factors involved in the risk of breast cancer. This study aimed to determine the effect of different environmental factors in the incidence of breast cancer in patients admitted in Imam Khomeini Hospital, Tehran. Estrogen hormone, age, weight, Dietary habits, vitamins, cigarette smoking and family history were concerned as risk factors.

Materials and Methods. The data were analyzed using the computer software SPSS for windows (version 11). The case-control study was conducted in 100 breast cancer patients and 100 healthy people as control.

Results. An increased association was found with patients becoming pregnant in high age, being pregnant for more twice in life period and who have had abortion. Using anti Pregnancy pills also is an important factor in the risk of breast cancer as a huge number of patients were using these kinds of tablets. Being over-weight also is a risk factor for breast cancer. Very few patients were using vitamins as it is an agent to reduce the risk of cancer. Positive association was also found among people using red meat and the number of using in the weak. No association was observed with age and risk of breast cancer. Factors like tea consumption and using fast food also resulted in positive association with breast cancer. We found no statistically significant association between positive family history and breast cancer risk.

Conclusion. This study shows that some of the environmental factors are important in increasing the risk of breast cancer.