New research states that women who have experienced traumatic or stressful events are twice as likely to have their cancer recur compared to other women. Women with stressful events in their life saw a recurrence after about 2.5 years. Women with more peaceful lives saw a recurrence of about five years. Stress has been shown to change the immune system’s function, altering the activity of natural killer cells. Researchers believe that this is related to breast cancer progression. Research between the stress and cancer development has usually produced mixed results. Cortisol, which is made in the adrenal glands during times of stress, may alter the body’s normal rhythms and potentially reduce resistance to tumor growth. An imbalanced cortisol output can also lead to estrogen dominance, an important factor behind breast cancer.