To investigate important factors with respect to health-related quality of life (HRQOL) in patients with longstanding insulin dependent diabetes mellitus, a patient cohort was followed prospectively for 10 years. The degree of metabolic control and the presence of late complications was assessed and HRQOL was measured with a 61-item questionnaire (SWEDQUAL) in which reference values have been obtained in a population sample. The results indicate that diabetic patients (n = 108) experienced a quality of life as good as a general population. When patients were divided into four groups based on metabolic control, those with poor control (HbA1c > or = 9%) rated their physical and emotional functioning significantly lower than those with lower HbA1c values. Nearly 15% of the patients reported 1-5 hypoglycaemic episodes during the latest 6 months. Despite a lower HbA1c they rated their general health as being poorer than patients without severe hypoglycaemia. Of the 108 patients 39% appeared to be free from late complications of diabetes. These patients rated their general health as better than patients who already had developed late complications. We conclude that a satisfactory metabolic control with a minimum of hypoglycaemic episodes is desirable not only to prevent late complications but also because poor metabolic control seems to be one reason why diabetic patients experience a poorer quality of life.