Using data from the Walnut Creek Contraceptive Drug Study (a prospective study begun in 1969 and continuing to the present), Minnesota Multiphasic Personality Inventory-(MMPI) measured scores for depression of 8932 women were studied in relation to the incidence of breast cancer. No statistically significant association between MMPI scores for depression and the subsequent development of breast cancer was found. There was neither an association of risk of breast cancer with repression/sensitization as measured on the MMPI nor with scores on the MMPI lie scale. This study is unique because it represents the largest reported prospective cohort in which the association between depression and breast cancer development has been examined.