BACKGROUND: Tetralogy of Fallot (TF) is a severe congenital defect of heart development. Fine-tuned sequential activation of Notch signaling genes is responsible for proper heart chamber development. Mutations in Notch genes have been associated with TF. The aim of this study was to analyze the activity of the Notch pathway in cardiac mesenchymal cells derived from ventricular tissue of TF patients. METHODS: Cardiac mesenchymal cells were isolated from 42 TF patients and from 14 patients with ventricular septal defects (VSDs), used as a comparison group. The Notch pathway was analyzed by estimating the expression of Notch-related genes by qPCR. Differentiation and proliferation capacity of the cells was estimated. RESULTS: The TF-derived cells demonstrated a dysregulated pattern of Notch-related gene expression comparing to VSD-derived cells. Correlation of Notch signaling activation level by HEY1/HES1 expression level with proliferation and cardiogenic-like differentiation of cardiac mesenchymal cells was observed but not with clinical parameters nor with the age of the patients. CONCLUSIONS: The data suggest a contribution of dysregulated Notch signaling to the pathogenesis of tetralogy of Fallot and importance of Notch signaling level for the functional state of cardiac mesenchymal cells, which could be critical considering these cells for potential cell therapy approaches.