In poorly regulated patients with type 1 diabetes mellitus (DM1), hepatic complications were sporadically observed and two entities were most commonly found: glycogen hepatopathy (GH) and non-alcoholic fatty liver disease (NAFLD) / steatohepatitis (NASH). The aim of the study was to determine the prevalence of hepatic complications in children and adolescents with DM1 and detect them, and to investigate if there is a correlation with chronic complications or associated autoimmune diseases. Also, one of the goals was to determine if GH and NAFLD could be differentiated non-invasively with MRI. We performed laboratory and ultrasound screening for the presence of liver disease in patients suffering from DM1 for more than a year. In those with detected changes, we performed further laboratory testing to rule out other potential liver disease. We performed liver MRI and biopsy in selected patients. We found a significantly higher prevalence of elevated transaminases and pathological ultrasound findings than previously described in the literature: 9.7% had clinically established hepatomegaly, 18.8% elevated transaminases, 21.7% ultrasound-enlarged liver and 27.3% changes in echogenicity. Liver MRI showed a good correlation with pathohistological findings and could be used as a non-invasive method to distinguish fat from glycogen accumulation. According to histological findings, 8 patients had GH, and only one NAFLD, which confirms that GH is by far the most common liver disease in patients with DM1. We did not establish correlation of hepatic complications with chronic complications or associated autoimmune diseases. In conclusion, hepatic complications in patients with DM1 are more common than previously thought. We must actively search for them by combined laboratory and ultrasound screening and try to determine their significance during long-term monitoring.