Multisystem inflammatory syndrome in children (MIS-C) is an immune-mediated complication of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. Cardiovascular system is commonly involved. Acute heart failure (AHF) is the most severe complication of MIS-C, leading to cardiogenic shock. The aim of the study was to characterise the course of MIS-C with a focus on cardiovascular involvement, based on echocardiographic (echo) evaluation, in 498 children (median age 8.3 years, 63% boys) hospitalised in 50 cities in Poland. Among them, 456 (91.5%) had cardiovascular system involvement: 190 (48.2%) of patients had (most commonly atrioventricular) valvular insufficiency, 155 (41.0%) had contractility abnormalities and 132 (35.6%) had decreased left ventricular ejection fraction (LVEF < 55%). Most of these abnormalities improved within a few days. Analysis of the results obtained from two echo descriptions (a median of 5 days apart) revealed a >10% increase in LVEF even in children with primarily normal LVEF. Lower levels of lymphocytes, platelets and sodium and higher levels of inflammatory markers on admission were significantly more common among older children with contractility dysfunction, while younger children developed coronary artery abnormality (CAA) more often. The incidence of ventricular dysfunction might be underestimated. The majority of children with AHF improved significantly within a few days. CAAs were relatively rare. Children with impaired contractility as well as other cardiac abnormalities differed significantly from children without such conditions. Due to the exploratory nature of this study, these findings should be confirmed in further studies.