Identification of electromagnetic fields in the vicinity of high voltage overhead transmission lines requires taking measurements and determining, on their basis, the maximum values of electric (Emax) and magnetic (Hmax) field strength, as well as the width of the area with electric field strength exceeding 1 kV/m. Measurements should be carried out in the places, where the distance between phase wires and the ground is smallest, and their results converted to conditions expected at the most disadvantageous parameters of the work of line in terms of their effect on the environment. This requires taking into consideration the maximum voltage of the transmission line, as well as the highest acceptable temperature of wires. In a single-circuit line, convertion of the measured values to expected maximum values of Emax and Hmax is not difficult, although it requires knowledge of cross-profiles in spans, in which measurements were taken. However, recounting the values of magnetic field strength produced by two-circuit lines creates a serious problem. Since in the majority of cases, the measurements in the surrounding of two-circuit lines are carried out at asymmetrical currents in both circuits, converting their results to symmetrical currents of the lines is practically impossible. Therefore, the results of the measurements of magnetic field strength carried out at the asymmetrical current of a two-circuit line cannot provide the basis for assessing the possibility of exceeding the allowable value of magnetic field intensity determined in the binding regulations. Such an assessment can only be done on the basis of the results of relevant calculations. Med Pr 2009;60(2):159-165