A climatology of the relationship between explosive cyclogenesis and atmospheric blocking has been constructed from an investigation of sea level pressure and 500-mb height analyses over the Northern Hemisphere during seven winter seasons. Planetary-scale 500-mb geostrophic u and v wind components and component anomalies (u′ and v′) were calculated over each explosive cyclone center and compared to the times and locations of block onsets. Blocking episodes were defined by the persistence, for at least 5 days, of negative zonal index (500-mb height at 40°N less than that at 60°N) spanning at least 18.75° of longitude. Thirty-three percent (43%) of the explosive cyclones that were not analyzed during blocking episodes over the Atlantic (Pacific) sector of the Northern Hemisphere and over which there were anomalously strong planetary-scale southerly winds (v′ > 0), anomalously weak westerly winds (u′ < 0), and v > u/2 were followed, in not more than 5 days and within 60° of longitude, by the onset of blocking in that sector. These percentages significantly exceed, at the 90% (95%) confidence level, the 12% unconditional probability of an unblocked day in either sector being followed in not more than 5 days by the onset of blocking in that sector. Conversely, while each of the block onsets was preceded, by not more than 5 days, by at least one sea level cyclone analyzed within 60° of longitude, in only 56% of these cases was one of the antecedent cyclones rapidly intensifying. In only 24% of the cases was block onset preceded (within 5 days and 60° of longitude) by explosive cyclogenesis over which v′ gt; 0, u′ < 0, and v/u > 0.5. An empirical rule developed from these findings may thus fail to predict many block onsets. The most common block-onset precursor, noted in 76% of the cases, was anomalously strong planetary-scale southerly winds over the antecedent cyclone. Statistically, the most significant feature distinguishing explosive cyclones followed by blocking from other explosive cyclones was weaker planetary-scale westerlies relative to normal over the block-onset cyclones. It is hypothesized that the onset of blocking in at least some of these cases was due to the anomalous planetary-scale advection of synoptic-scale potential vorticity associated with the antecedent cyclones or to the interaction between synoptic-scale processes and “preconditioned” (anomalously amplified) planetary waves.