The regional melt onset signal of Lagrangian elements of Arctic sea ice in RADARSAT synthetic aperture radar (SAR) data during the spring of 1998 is examined. The melt signal is clearly detectable not only in backscatter changes over multiyear ice but also first-year ice and mixtures of these ice types. This allows a more complete mapping of the progression of melt over the sea ice cover. For the spring of 1998 the onset dates range between 10 May and 29 June. The spatial pattern of melt onset is characterized by sharp boundaries delineating distinct regions with relatively uniform onset dates. This pattern appears to be associated with moisture and warm air brought in by a low-pressure trough and the timing of a rain event reported at the Surface Heat Budget of the Arctic Ocean (SHEBA) camp. Measurements from the SHEBA camp show good correspondence between the timing of melt in SAR imagery and the onset of albedo drop off associated with the beginning of summer. Onset dates are compared with the timing of the zero crossing of the temperature records from drifting buoys and the onset dates derived from satellite passive microwave brightness temperature fields. The timing of our estimates is within 1--2 days of the zero crossing of the buoy temperature records. Onset dates derived from passive microwave observations appear to be biased toward a later stage of melt. Comparison with results derived from a SAR data set from the spring of 1992 show that the onset of melt in 1998, derived from RADARSAT, in the Beaufort Sea occurred 2 weeks earlier.