The GPS radio occultation (RO) soundings by the FORMOSAT-3/COSMIC (Taiwan¡¦s Formosa Satellite Misssion #3/Constellation Observing System for Meteorology, Ionosphere and Climate) satellites launched in mid-April 2006 are compared with high-resolution balloon-borne (radiosonde and ozonesonde) observations. This paper presents preliminary results of validation of the COSMIC RO measurements in terms of refractivity through the troposphere and lower stratosphere. With the use of COSMIC RO soundings within 2 hours and 300 km of sonde profiles, statistical comparisons between the collocated refractivity profiles are erformed for some tropical regions (Malaysia and Western Pacific islands) where moisture-rich air is expected in the lower troposphere and for both northern and southern polar areas with a very dry troposphere. The results of the comparisons show good agreement between COSMIC RO and sonde refractivity rofiles throughout the troposphere (1 - 1.5% difference at most) with a positive bias generally becoming larger at progressively higher altitudes in the lower stratosphere (1 - 2% difference around 25 km), and a very small standard deviation (about 0.5% or less) for a few kilometers below the tropopause level. A large standard deviation of fractional differences in the lowermost troposphere, which reaches up to as much as 3.5 - 5%at 3 km, is seen in the tropics while a much smaller standard deviation (1 - 2% at most) is evident throughout the polar troposphere.