BACKGROUND: COVID-19 pandemic caused families to stay home and cancel everyday activities. Hospital admissions decreased, affecting changes in diagnoses and management of chronic disease in children. AIMS: We analyzed how the first lockdown influenced clinical presentation and manifestation of children with diabetes mellitus (DM) in a German University Hospital. METHODS: During March 15th and October 11th 2020, data on general patient information, clinical symptoms and on lab results related to diabetic ketoacidosis (DKA) were analyzed in children (0-18years) who presented with new onset of DM or poor metabolic control of known DM. All data including frequency and severity of DKA were compared to data from patients who presented in 2019. RESULTS: Data from 125 participants with DM were evaluated (2020: n=52; 2019: n=73). In 2020, twelve patients (23.1%) were diagnosed with new onset DM, two of them with type2 diabetes, and 66.7% presented with DKA including both patients T2DM. In 2019, 24.5% of patients had new onset DM, and 50% of them presented with DKA. In 2020, patients with new onset DM were younger, presented with more severe symptoms of DKA and had to stay longer in hospital compared to 2019. In 2020, six children (50%) with new onset DM were <6years, whereas in 2019 most children with new onset DM were adolescents (n=7, 38.9%). CONCLUSION: COVID-19 lockdown aggravated complications of diabetes onset and therapy management, including severity and frequency of DKA. It underlines the need of health education for early DKA diagnosis to early identify children at risk.