Diabetes mellitus is a chronic disease characterized by the inability of the body tomaintain normoglycemia. Treatment of diabetes relies mostly on diabetes self-management,requiring a large investment of time and energy on a daily basis. Psychologicalwellbeing, behavioral patterns and social context play a major role in diabetesself-management and glycemic control. Social isolation behavior (self-quarantining) mayimpact glycemic control by influencing daily routines, therapy adherence, physicalactivity, and self-measurement and eating behaviors. Therefore, a period of nationwideself-quarantine, such as during the lockdown issued during the COVID-19 outbreak in theNetherlands, may have a large effect on glycemic control in patients with diabetes.In this observational cross sectional study, we aim to assess the impact of long-termself-quarantine on glycemic control, diabetes self-management and distress in patientswith type 1 and type 2 diabetes mellitus.A specific subgroup of patients with T1D are those with complicated diabetes who havereceived a pancreas or islet transplantation and use immunosuppression, having multiplerisk factors for severe COVID-19. The impact of lockdown strategies on mental andphysical health is expectedly even greater in patients at even higher risk for severeCOVID-19. We therefore additionally investigated differences in behavioral, mental andphysical implications of a nationwide lockdown on patients with type 1 diabetes with andwithout islet or pancreas transplantation.Measurements will be performed during the lockdown period. Patients will be asked toperform a fingerprick HbA1c measurement once, sent back to the LUMC by mail. Data fromcontinuous or flash glucose monitoring devices will be collected according to standardclinical practice. Furthermore, patients will be asked to fill out an onlinequestionnaire once on diabetes self-management behavior, well-being and distress, alongwith questions about health status, level of education, medication use, employment,social situation and the impact of self-quarantine on daily routines. In thisquestionnaire, we ask patients to compare certain aspects of their life (e.g. anxiety,stress, weight, physical activity, glycemic control) at the time of the lockdown tobefore the lockdown. Data on demographics, type of diabetes, weight, BMI and HbA1c priorto the COVID-19 outbreak will be derived from the patient's electronic health file.