Diabetes mellitus (DM) is a frequently encountered metabolic disease with chronic features and involves numerous complications throughout its course, which causes severe restriction and disability in an individual's life. It has been reported that the incidence of depression is higher in diabetic patients and that diabetes is one of the risk factors in the development of depression. It has also been reported that co-morbid psychiatric disorders cause further deterioration in the quality of life in diabetic patients. The aim of this study was to investigate the effects of depression on the quality of life in type II DM patients. Sixty patients (30 females and 30 males) with current major depressive episode diagnosed according to DSM-IV criteria, and 48 type II DM patients (30 females and 18 males) without a major depressive episode (non-depressed group) were included in the study. All patients were evaluated with a semi-structured interview form to assess the clinical features of DM, Hamilton Rating Scale for Anxiety (HRSA), Hamilton Rating Scale for Depression (HRSD), and the Turkish version of The World Health Organization Quality of Life Assessment-Brief (WHOQOL-BREF). The HRSD and HRSA scores in the depressed group were 24.87 +/- 4.83 and 21.07 +/- 5.44, respectively, whereas those in the non-depressed group were 7.83 +/- 3.92 and 6.88 +/- 3.43, respectively. The physical health, psychological health, social relationship, environmental and social pressure domain, general health-related quality of life, overall quality of life, and WOQOL-BREF total scores were found significantly lower in the depressed group than the non-depressed group. There were significant negative correlations between HRSD and HRSA scores and physical health, psychological health, social relationship, environmental and social pressure domain, general health-related quality of life, overall quality of life, and WHOQOL-BREF total scores. Furthermore, there were significant negative correlations between the HbA1c level and physical health, social relationship, environmental domain, social pressure domain, general health-related quality of life, overall quality of life, and WHOQOL-BREF total scores. However, there was a significant positive correlation between the level of education and physical health, psychological health, social relationship, environmental social pressure domain, overall quality of life, and WHOQOL-BREF total scores. There were significant negative correlations between social relationship domain score, and age and duration of illness. Our study demonstrates that the presence of depression in type II DM further deteriorates the quality of life of the patients. Since treating depression would have a beneficial effect on the quality of life, clinicians should carefully assess for depression associated with type II DM.