Satellite passive-microwave data for November 1978 through December 1996 reveal marked seasonal, regional, and interannual variabilities, with an overall decreasing trend of −34,300±3700 km2/yr (−2.8%/decade) in Arctic sea ice extents over the 18.2-year period. Decreases occur in all seasons and on a yearly average basis, although they are largest in spring and smallest in autumn. Regionally, the Kara and Barents Seas have the largest decreases, at −15,200±1900 km2/yr (−10.5%/decade), followed by the Seas of Okhotsk and Japan, the Arctic Ocean, Greenland Sea, Hudson Bay, and Canadian Archipelago. The yearly average trends for the total, the Kara and Barents Seas, and the Seas of Okhotsk and Japan all have high statistical significance, with the null hypothesis of a 0 slope being rejected at a 99% confidence level. Regions showing increasing yearly average ice extents are Baffin Bay/Labrador Sea, the Gulf of St. Lawrence, and the Bering Sea, with only the increases in the Gulf of St. Lawrence being statistically significant at the 99% level. Hemispheric results for sea ice areas exhibit the same −2.8%/decade decrease as for ice extents and hence a lower absolute decrease (−29,500±3800 km2/yr), with the ice-free area within the ice pack correspondingly decreasing at −4800±1600 km2/yr. Confidence levels for the trends in ice areas and ice-free water areas exceed 99% and 95%, respectively. Nonetheless, interannual variability is high, and, for instance, the Arctic Ocean ice extents have a positive trend 1990–1996, in spite of their negative trend for the time period as a whole.