Background: At present, there are no methods for intraoperative monitoring of the interventricular septum (IVS) thickness in a stopped and empty heart. This might be an obvious reason for unsatisfactory results after a number of septal myectomies.Aim: To provide an experimental background for the method to control the IVS thickness (that we had proposed) during septal myectomy.Materials and methods: The proposed technique is based on the transillumination method. The experimental models were cadaveric porcine hearts, as well as fragments of the human myocardium removed during septal myectomies. The thickness of the translucent myocardium was estimated depending on the local illumination value at the entrance to the myocardium and the external illumination of the surgical field. We compared the results of 67 septal myectomies performed in the clinic of the Almazov National Medical Research Centre with the results of 35 similar experimental procedures with cadaveric porcine hearts using the proposed way of measurement.Results: A graph of the illumination at the entrance to the myocardium against the thickness of the translucent myocardium was constructed. After conventionally performed septal myectomies the median variation of the myocardial thickness was 4 [3; 6] mm. In the experiment using the proposed control method, the median variation was 1 [1; 2] mm, i.e. significantly less than with the conventional approach (p = 3 x 10-10).Conclusion: The method to control the IVS thickness when performing septal myectomy makes it possible to achieve the required myocardial thickness the resection area with much greater accuracy than with the conventional one.