This paper summarizes similarities and differences between two cohorts of the National Treatment Improvement Evaluation Study (NTIES) clients, those age 45 or older and those younger than 45. Results reveal the following: Clients in the 45+ cohort were considerably more likely to be treated for problems with alcohol or with heroin, and they were correspondingly less likely to be treated for crack, cocaine, or other drugs; for clients in the 45+ cohort who were treated for heroin, it appears that methadone treatment programs did not work well; of those treated for alcoholism in this group, post-treatment relapse rates were 40-50%; the overall percentage of 45+ clients who reported being troubled by their alcohol use declined from 30% to 10% following treatment; needle use and needle sharing declined sharply after treatment; physical disabilities were more prevalent among the 45+ group; and the 45+ cohort members evidenced marked pre-treatment versus post-treatment declines in the prevalence and amount of physical assaults and criminal activities, as well as in the frequency of arrests for such behaviors, with reductions of around 80% being seen for several indicators. An appendix entitled "Description of the National Treatment Improvement Evaluation Study and Center for Substance Abuse Treatment Demonstrations (1990-1992)" is included. (Contains 3 figures and 15 tables.) (MKA)