There is considerable support among members of the public and health professionals for the view that depression is an aetiological factor in the development of cancer. We review evidence for this hypothesis based upon prospective longitudinal studies measuring depression at time 1 and subsequent incidence of cancer over the intervening period to time 2. A meta-analysis of available studies indicates a small, but marginally statistically significant association between depression and the later development of cancer. However, the practical significance of this association for the prevention of cancer is negligible.