Assessment of a region's seismic hazard requires knowledge of the repeat times and the regularity of earthquake recurrence. For most areas, the earthquake catalog is too short to determine recurrence parameters reliably. Using a large set of events for many regions however, it should be possible to constrain the average characteristics of earthquake recurrence behavior. This study compiles a set of the most complete historic and paleoseismic series of M≥7 earthquakes from the Middle American Trench, Alaska, Chile, Japan, and the San Andreas Fault. The 52 recurrence series show a large average aperiodicity, that is, variations in recurrence time are larger than 40% of the average recurrence interval. Aperiodicity may decrease with increasing magnitude. Neither the use of different magnitude thresholds, the exclusion of foreshocks and aftershocks, the type of statistics (normal or lognormal) used, nor the choice of segmentation results in a significantly smaller aperiodicity. The irregular recurrence behavior exhibited by the data implies that earthquakes are more unpredictable than often assumed in hazard analyses. ¿ American Geophysical Union 1996