Structural equation modeling with multiple groups was used to examine relationships among pretreatment patient characteristics, treatment retention, and treatment outcomes among younger and older adults in the Drug Abuse Treatment Outcome Studies. Separate models were tested for 551 patients treated in long-term residential (LTR) programs and 571 patients treated in outpatient drug-free (ODF) programs. There was a stronger positive relationship between treatment retention and abstinence at follow-up for younger adults in both treatment modalities. Prior treatment history had a negative effect on self-efficacy to resist drug use for older adults in LTR. Negative reference group influence was reduced for all groups following treatment, however, it was more strongly related to abstinence for younger adults in LTR and for older adults in ODF. Clinical implications of age-related differences in these relationships are discussed.