The verification of the diagnosis of “myocarditis”, despite the active introduction of modern survey methods, is still complicated, which leads not only to late diagnosis, but also to worsening the prognosis of patients. The purpose of this study was to identify the causes of late diagnosis of inflammatory diseases of the myocardium, evaluate the factors influencing the course of the disease, and the effectiveness of the therapy in patients with morphologically documented myocarditis. The study involved 50 patients with morphologically documented myocarditis. The duration of follow-up was 3.5±1.6 years. In 59% of cases, myocarditis was diagnosed within a period exceeding 6 months of the first clinical and instrumental manifestations. Long-term prognosis of patients with regard to global contractility of the left ventricle depended on the timing of verification of the diagnosis (c² = 6.9, p <0.05) and the clinical variant of the debut of the myocarditis (c² = 13.7, p <0.01). There was a significant deterioration in the prognosis in patients who have a heart failure clinic and rhythm disorders in the onset of the disease, as well as in patients with late diagnosis of the disease due to erroneous treatment. It was concluded that the presence of idiopathic rhythm disturbances should be considered as an excuse for excluding myocarditis in patients with already diagnosed heart diseases. The study proved an improvement in the prognosis on the background of combined therapy for heart failure (ACE inhibitors, beta-blockers and diuretics) (c² = 9.28, p <0.01), and statin therapy (c² = 6.04, p <0.05).