Objective: To examine psychological functioning and self-management behaviours of Dutch adult patients with insulin-requiring diabetes mellitus suffering from extreme fear of self-injecting (FSI) and/or fear of self-testing (FST). Methods: A cross-sectional survey was performed in a sample of insulin-treated diabetes patients (n = 1275; 51.1% male; age 49.7 ± 15.8 years; 58.0% Type 1 diabetes), assessing FSI and FST. Patients completed the questionnaires concerning trait/state anxiety, depression, fear of hypoglycemia, diabetes-related distress, diabetes self-care activities, and general well-being. Comparisons were made on these measures between patients with extremely high scores on FSI and/or FST (≥ 95th percentile) and the other patients. Patients with extreme scores on FSI and/or FST were invited to take part in a second survey to assess the prevalence of major depression, common fears/phobias, and psychoneuroticism. Results: People with extreme FSI/FST scores, as compared to the other patients, reported higher levels of trait/state anxiety and depression. This group also reported more fear of hypoglycaemia and diabetes-related distress, had lower levels of general well-being, and reported less frequent self-monitoring of blood glucose. The second survey showed 11.1% of patients with extreme FSI/FST reporting scores indicating major depression. Prevalence of scores greater than or equal to the high scores on phobias (38.0-63.3%) and psychoneuroticism (27.8%) were consistently higher than norm group prevalences. Discussion: Extreme levels of FSI and/or FST are associated with high diabetes-related distress, poor general well-being, and psychological comorbidity, as well as poorer adherence to the diabetes treatment regimen. It is concluded that patients with extreme FSI/FST are often burdened with more than this specific phobia.