We developed a lifetime severity index for cocaine use disorder and examined its predictive validity of posttreatment outcome using data from the national Drug Abuse Treatment Outcome Study. The index, based on 28 items, considered frequency of use, recency, dependency, and attempt to quit. A higher value of the index, indicating greater severity, predicted a greater likelihood of relapse (the odds ratios were 5.7 for high severity and 4.4 for medium severity, relative to low severity) and shorter time to relapse. Similarly, the polytomous logistic analysis indicated that the index predicted levels of posttreatment cocaine use (odds ratios of daily use were 47.8 for the high severity and 18.8 for medium severity; the corresponding odds ratios of weekly use were 6.75 and 5.10 and for less-than-weekly use were 3.35 and 3.57). The index can be a useful measure for both clinical and research purposes.