A high noisy environment severely contaminates the ECG signal. In this paper, the use of a wavelet transform domain filtering technique as an adaptive de-noising tool, implemented in ECG signal analysis, is presented. The multi-resolution representations of the signal, produced by wavelet transform (WT), are used for signal structure extraction. Experimental results have shown that the implementation of this wavelet-based filter in ECG signal analysis results in an efficient reduction of the high noisy environment, producing an almost noise-free output signal. Due to its simplicity and its fast implementation the method can easily be used in clinical medicine.