A variety of indexes are constructed based on weights derived from Sellin and Wolfgang's 'seriousness' scale. The results reveal that indexes weighted by offense seriousness are not very highly correlated across data sources for a sample of twenty-six American cities. The substantive importance of the discrepancies between UCR and NCS estimates is illustrated by examining the ecological correlates of weighted crime rates for the twenty-six city sample and by assessing changes in weighted crime rates at the national level for the 1973-81 period. The results also reveal that if estimates of homicide are included in the composite indexes by supplementing NCS data from the Center for Health Statistics, than under certain weighting conditions the correlations across data sources are nearly perfect. The paper concludes with a discussion of the implications of this latter finding for the measurement of overall levels of criminal activity and for the scaling of the relative gravity of different offenses. (Author abstract)