Various hypotheses have been proposed to account for the relationship between CFS and Depression, including the following:- 1.) The view of the CFS as an atypical manifestation of Major Depressive Disorder (MDD). Further, it has been proposed that somatisation and alterations in the Central and Autonomic nervous systems may underlie the somatic expression of psychological symptoms suggested by this hypothesis. 2.) The Depression- Vulnerability Hypothesis proposes that CFS occurs in individuals with a pre-morbid vulnerability to depression and the prolonged disability associated with CFS may be attributable to MDD. 3.) The hypothesis that depression may occur as the result of CFS and may represent a reaction to loss of health or a psychiatric disorder of organic origin. This study investigated current psychiatric morbidity and the pre-morbid prevalence of psychiatric disorder in a sample of thirty-five patients with CFS. Seventy-one per cent of this sample satisfied criteria for current psychiatric disorder. Fifty-four per cent satisfied criteria for depressive disorder, and a further thirty-seven per cent satisfied criteria for an anxiety disorder. The pre-morbid prevalence of psychiatric disorder and MDD was forty-two per cent and thirty-one per cent respectively. These results are comparable to those found in previous studies. Attribution of symptoms by CFS patients to external and internal causes was also investigated and the relationship between attributional style and depressive symptomatology was explored. The relationship between depression and CFS was further explored by comparison of the CFS sample with a sample of thirty- three patients with Major Depressive Disorder or Dysthymia, in terms of subjective fatigue, personality factors and dimensions of family functioning (family of origin). Subjects in the CFS sample with concurrent depression formed a sub- group for the purpose of statistical analysis.