The aims of this research were: 1. To examine the role of adverse life events in the response of depressed patients to treatment; 2. To examine the relative effectiveness of cognitive therapy and biological treatment when patients experience life events; and 3. To examine the mediating variables of dysfunctional attitudes, attributional style and coping style in patients receiving cognitive therapy and patients receiving biological treatment and the effects of life stress on these processes. Cognitive therapy did not lead to more effective coping than biological treatment. The main hypothesis was not supported. However, it seems that cognitive therapy and biological treatment operate in different ways as can be seen by their differing effects on dysfunctional attitudes and attributional style. The negative findings may be due to the small sample size and the fact that few patients experienced major life events. Larger numbers may have resulted in more powerful statistical effects. The results need to be replicated with larger numbers. The findings are discussed with reference to the literature. The implications of the findings are that 1) there may be no advantage in treating depressed patients who experience life events with cognitive therapy instead of medication; 2) cognitive therapy may need to be more tailored towards the social context in which depression occurs in order to increase its effectiveness; 3) patients receiving cognitive therapy appeared to cope equally with life events compared to patients receiving biological treatment which suggests that coping style may not be a mechanism through which cognitive therapy reduces depression. It is possible rather that improvements in coping occur as a result of reduced levels of depression.