This research was aimed to demonstrating to the pediatric practitioners of a wide range the clinical case of retrospective diagnosis of Kawasaki disease (KD) based on detection of coronary artery (CA) aneurysms and anamnestic data analysis. Mucocutaneous lymphonodular syndrome (KD) is a systemic vasculitis of childhood that was characterized in the observed clinical case by fever, inflammatory changes in the mucous membranes, skin, lymph nodes and CA damage. Features of this clinical case were as follows: 1. the diagnosis was established retrospectively 5 years later than the KD was suffered based on accidental detection of CA aneurysms, 2. the lack of adequate KD treatment had led to the formation of CA aneurysms, including a single one of a giant size, 3. despite the long-term absence of generally accepted thrombosis prophylaxis in a patient with a giant CA aneurysm, only 11 years later did a thrombus in the aneurysm formed for the first time, which had then regressed rapidly despite it is known from the bibliographical data and those of the Authors’ own that the risk for thrombosis is very high in patients with giant aneurysms. Conclusion: late KD diagnosis entails the development of the cardiovascular system complications, in particular, coronary aneurysms, which in its turn in the reported clinical case were an accidental finding during the patient’s checkup.