Self-reported stress levels predict subsequent breast cancer in a cohort of Swedish women.

The association between stress and breast cancer has been studied, mostly using case-control designs, but rarely examined prospectively. The purpose of this paper is to describe the role of stress as a predictor of subsequent breast cancer. A representative cohort of 1,462 Swedish women aged 38-60 years were followed for 24 years. Stress experience at a baseline examination in 1968-69 was analysed in relation to incidence of breast cancer with proportional hazards regression. Women reporting experience of stress during the five years preceding the first examination displayed a two-fold rate of breast cancer compared with women reporting no stress (age-adjusted relative risk 2.1; 95% CI [1.2-3.7]). This association was independent of potential confounders including reproductive and lifestyle factors. In conclusion, the significant, positive relationship between stress and breast cancer in this prospective study is based on information that is unbiased with respect to knowledge of disease, and can be regarded as more valid than results drawn from case-control studies.