Although quality of life evaluations are widely used throughout medicine, relatively little is known about the psychological processes involved in making these judgements. What is known is that quality of life judgements are not straightforwardly associated with objective measures of health status or clinician ratings. In particular, patient affect appears to be associated with quality of life ratings but whether this relationship is secondary to physical health or not is controversial. The aim of this study was to determine the role of anxiety and depression in the reporting of quality of life in a group of patients with diabetes mellitus. One hundred consecutive patients with diabetes (insulin-dependent diabetes mellitus n = 36 and non-insulin-dependent diabetes n = 64) were assessed using a self-report measure of quality of life, a symptom checklist and a questionnaire measure of anxiety and depression. In addition, they were independently rated for their level of physical impairment. The results showed that depression and, to a lesser extent, anxiety were significantly related to self-reported quality of life even when the differences in physical health and age were controlled for statistically. This study shows that, independent of the level of physical illness, affect, particularly depressive affect, is an important factor in the determination of quality of life.