Objective: Multisystem inflammatory syndrome in children (MIS-C), is a newly recognised life-threatening complication of coronavirus disease 2019 (COVID-19). Early determination of clinical severity of the disease is important for early decision of treatment regimens. The aim of this study is to investigate the severity classification value of a number of hematological parameters, inflammatory markers and biochemical tests in patients with MIS-C during the acute stage and after anti-inflammatory treatment. Material and Methods: In this retrospective case-controlled study, 64 children with MIS-C and 95 healthy age and gender matched children were included. Patients were divided into three clinical severity groups; mild, moderate, and severe. Results: Mean platelet volume (MPV), MPV to lymphocyte ratio (MPVLR), d-dimer, ferritin, interleukin 6 (IL-6) levels were significantly higher, while albumin levels were lower in the severe MIS-C group compared to all the other groups on admission. Neutrophil-to-lymphocyte ratio (NLR) and derived (d) NLR (d-NLR) levels were significantly higher in the moderate group compared to the mild group. In the pre-treatm ent period of MIS-C patients had higher MPV, platelet distribution width (PDW) values while they had lower white blood cell, lymphocyte, monocyte, haemoglobin, mean corpuscular volume (MCV), red cell distribution width (RDW), plateletcrit and platelet values compared to the post-treatment group. Lymphocyte, platelets, and haemoglobin levels were significantly higher in the control group compared to the pre-treatment group. Acute phase reactants, NLR, NMR, PLR, d-NLR, MPVLR and systemic inflammatory index were significantly higher in all MIS-C patients on admission compared to the control group. Conclusion: Specific routine laboratory test results may be useful in determining disease severity of MIS-C, possibly predicting the prognosis and early initiation of the appropriate treatment.