Aerosol hygroscopicity is a property that reveals the ability of an aerosol particle to grow under increasing values of relative humidity. The hygroscopic behavior has a significant effect on radiative properties of aerosols, and therefore on cloud formation, aerosol-cloud interaction and, consequently, on the Earth’s climate. In this work, a Raman LIDAR is used to determine the hygroscopic growth factor fβ (RH) under unperturbed, ambient atmospheric conditions in a well-mixed boundary layer in São Paulo metropolitan city. To this aim, the water vapor mixing ratio (required to derive the hygroscopic growth factor) was independently obtained by radiosoundings and Raman LIDAR (after the corresponding calibration using radiosoundings), and the hygroscopic growth factor was determined using both instruments. There is a good agreement between the values obtained by the LIDAR and by the radiosoundings, although many uncertainties still remain in the hygroscopic growth factor determination. It suggests that the Raman LIDAR method can provide useful measurements of the dependence of aerosol optical properties on relative humidity and under conditions closer to saturation.