We exploit the cloud-penetrating capability and insensitivity to precipitation of the Global Positioning System radio occultation (GPSRO) technique to study the humidity environment of tropical cyclones (TCs). We focus in regions within and around the vicinity of TCs' eye, where infrared and microwave observations are difficult to acquire due to cloudiness and heavy precipitation. We use data from the National Hurricane Center TC Best Tracks to identify the location of North Atlantic TCs. The Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) Data Analysis and Archive Center provides concurrent refractivity, temperature, and humidity measurements from the Challenging Minisatellite Payload and COSMIC missions and the European Centre for Medium-Range Weather Forecasts (ECMWF). The distribution of GPSRO-derived humidity profiles as functions of altitude and distance from the storms' center identifies a pronounced low-level inflow, characteristic of mature TCs, at distances between 50 and 90 km, which coincide with the area where the eyewall of TCs forms. We also capture wavelike structures resembling spiral rainbands beyond the eyewall. The distribution of water vapor as functions of altitude and TC intensity shows a decrease at all altitudes when a tropical system matures from a tropical depression to a Category 1 (Cat 1) hurricane. The water vapor gradually increases from Cat 2 to Cat 5 hurricanes—a result that is also identified in ECMWF data sets and Atmospheric Infrared Sounder observations. We conclude that GPSRO data can contribute significantly to the understanding and modeling of the vertical structures of TCs.