We report the first continuous measurements of chlorine nitrate (ClONO2) in high-latitude regions taken by the Improved Limb Atmospheric Spectrometer (ILAS) on board the Advanced Earth Observing Satellite (ADEOS) and processed using the latest data retrieval algorithm (version 6.1). Performance of the measurements, validation with three balloon-borne sensors, and seasonal variation of ClONO2 in the Arctic and Antarctic stratosphere are presented, as well as a brief description of the version 6.1 algorithm and data characteristics for both the Arctic and Antarctic. Although the ILAS-measured ClONO2 data show, on average, ~30% lower values than the validation data, they agree with validation data within the combined total error (~20--40%) of the ClONO2 measurements at ~15- to 32-km altitudes. In the Arctic, enhancement of ClONO2 amounts was observed in spring 1997 after the appearance of polar stratospheric clouds (PSCs) inside the polar vortex. This is the result of preference for ClONO2 formation rather than HCl after the activation of ClOx in this Arctic spring of 1997. In the Antarctic, ClONO2 amounts showed strong local time/latitudinal dependence around the austral fall equinox in 1997.