Background: Myocarditis is commonly diagnosed in the intensive care cardiology unit (ICCU). No current recommendation nor guideline aids aetiological assessment. Methods: From September 2021 to October 2023, 84 patients with acute myocarditis underwent thorough and systematic serum and blood cell panel evaluations to determine the most common causes of the myocarditis. Results: Of the 84 patients (median age 34 years, range 22–41 years, 79% male), 16 presented with complicated myocarditis. The systematic aetiological assessment revealed that 36% of patients were positive for lupus anticoagulant, 12% for antinuclear antibodies, 8% for anti-heart antibodies, and 12% for anti-striated muscle antibodies. Viral serology did not yield any significant results. After the aetiological assessment, one patient was diagnosed with an autoimmune inflammatory disorder (Still’s disease). T-cell subset analyses indicated that myocarditis severity tended to increase with the T-cell lymphopenia status. Conclusions: A comprehensive, systematic aetiological assessment was of limited value in terms of predicting the clinical or therapeutic outcomes in myocarditis patients presenting to the ICCU.