A possible association between stress and risk of breast cancer has been assessed in different study designs with conflicting results. We prospectively followed 6,689 women from the Copenhagen City Heart Study for 18 years for a first-time diagnosis of primary breast cancer. The women were asked about stress intensity and frequency at baseline in 1981-83. Women reporting high levels of stress were at lower risk of breast cancer (hazard ratio = 0.60; 95% CI: 0.37-0.97) than women reporting low levels of stress. This association was most pronounced among women who received hormone therapy.