There is an absence of systematic, comparative research examining the negative consequences that are experienced as a result of using specific substances. Further, techniques typically used for needs assessment (i.e., prevalence proportions) do not take into account the probability of experiencing a negative consequence as a result of using specific substances. An approximated severity index is proposed that: a) takes into account the probability of experiencing negative consequences as a result of using specific substances; and b) is comparable across substances. Data from the NSDUH and the ADSS are used to demonstrate these techniques. The findings suggest that substances typically considered priorities based on prevalence proportions are not the same substances that have a high probability of causing negative consequences. The policy implications of these findings are discussed. (Contains 3 tables and 2 footnotes.)