More than ten years ago, the development of a new kind of semiconductor laser in the mid-infrared region, known as quantum cascade laser (QCL)[1], opened new interesting perspectives for basic research in molecular physics, as well as for applications to other scientific domains [2], mainly because all simple molecules exhibit strong fundamental vibrational bands in that spectral window. Nowadays, the QCLs’ potential is increasing more and more with the significant advances of the QCLs’ fabrication technology, which has allowed to lower the emission wavelength below the limit of 5 µm [3] and bring the operation temperature as close as to room temperature [4], still preserving the continuous wave operation. A further frontier of QCLs is given by the operation in the terahertz frequency range [5].The present work deals with high resolution molecular spectroscopy by means of a new, continuous wave, liquid-nitrogen cooled, distributed feedback QCL at a wavelength of 4.3 µm, expressly built for us by the company Alpes Lasers. In particular, we report on Lamb-dip spectroscopy for carbon dioxide by means of a free-running QCL. Combining the simple pump-probe scheme with wavelength modulation spectroscopic technique (WMS), we observed the sub-Doppler profile of CO2 lines belonging to the ν2 1+ ν3-ν2 1 hot band. A number of laboratory tests were performed in different experimental conditions in order to quantify the different contributions to the width of the sub-Doppler lines. For this purpose, the WMS spectra were carefully analysed by means of a fitting procedure that was based on the Fourier expansion of a Lorentzian profile, also …