The pressure sensors on balloon-borne sondes relate the sonde measurements to height above the earth's surface through the hypsometric equation. It is crucial that sondes used to explore the vertical structure of the atmosphere do not contribute significant height errors to their measurements of atmospheric constituent concentrations and properties. To describe quantitatively the magnitude of the error introduced by the pressure sensor, a series of radiosonde flights was conducted at Wallops Island, Virginia. In most cases, each flight consisted of two sondes attached to a single balloon; each flight was tracked by a highly accurate C-band radar. For the first 19 radiosondes, the standard aneroid cell-baroswitch assembly used by the National Weather Service was the pressure sensor. The last 26 radiosondes were equipped with a premium grade aneroid cell-baroswitch assembly sensor and with a hypsometer. Analysis has revealed that both aneroid cell-baroswitch sensors become increasingly inaccurate with altitude. At 35 km altitude, the standard deviation of the sonde sensor-radar differences was found to be 1.838 and 0.742 km, respectively, for the standard and premium sensors. On the other hand, the hypsometer-radar differences are not strongly dependent upon altitude, and the standard deviation of the differences at 34 km was found to be 0.276 km.