A new accurate determination of the spatial and temporal variations of northern wetland area extent, during the snow-free period, is retrieved from the 19- and 37-GHz passive microwave Special Sensor Microwave/Imager (SSM/I) brightness temperature database. The total fraction of water surface (FWS) area, corresponding to small lakes or reservoirs (area < 25 × 25 km2), inundated surfaces and natural wetland areas, computed on a weekly basis and averaged over the 1988–2001 period, gives a maximum summer extent of 10.10 ± 0.21 × 105 km2 for Canada and Alaska and 12.7 ± 0.35 × 105 km2 for Eurasia, above 50°N. The mean seasonal dynamics of wetland and inundated surfaces shows a maximum area extent during the months of July-August-September over North America (48°N–68°N) and between June and September over northern Eurasia. The interannual variations of the yearly total FWS area since 1988 show a slight increase (8% ± 3%) in Canada-Alaska, mainly due to a decrease in the snow cover extent. The FWS interannual variability in the Ob River basin, which is well explained by the water discharge variations, shows an increase of 8.5% ± 1.5% between 1988 and 2001. The microwave-derived FWS estimate and its dynamics present an interesting potential for studying the climate-driven variability of wetlands and inundated areas.