The purpose of the study was to find out which of five Quality of life (QoL) questionnaires are reliable and valid for evaluating the QoL in German patients with type 2 diabetes: the Diabetes Treatment Satisfaction Questionnaire (DTSQ), the Well-Being Questionnaire (WBQ), the Short Form SF36 of the Medical Outcome Study (SF36), the Diabetes39 and the Quality of Life with Diabetes (LQD) questionnaire. A stratified sample of 144 patients who were attending one of nine special hospitals for people with diabetes at the time of the study were given the questionnaires twice. Most of the scales of the questionnaires had internal consistencies (alpha) of above 0.80 as recommended; the retest reliabilities were lower, especially for scales evaluating satisfaction with levels of blood sugar and treatment in general. A factor analysis of all scale scores yielded four factors: (1) physical aspects, (2) well-being and satisfaction, (3) diabetes-specific stress, and (4) treatment satisfaction. Construct validity showed frequency of hypoglycemias, neuropathies, treatment with insulin and number of late complications to have a negative impact on QoL scores. No single questionnaire covered all relevant aspects of the QoL of subjects with type 2 diabetes. Thus, quality of life in diabetic people should be evaluated with scales representing the cognitive dimensions found in this study so as not to miss significant aspects.