Hierarchical linear models were used to assess the relationship of background factors and legal pressure with treatment participation for 90 days or longer. Two-thirds of the sample entered residential treatment with moderate to high pressure from legal authorities, and they were significantly more likely than the low-pressure clients to stay 90 days or more. Moreover, the difference in retention between moderate-to-high and low-pressure clients was even greater in programs with proportionately larger caseloads under legal surveillance. The criminal justice system can influence treatment participation and retention, and it is apparently essential for the criminal justice system and treatment programs to maintain an open and constructive relationship to maximize their combined impact. Whereas moderate levels of pressure are associated with greater program retention, more intensive supervision, especially after program discharge, might reinforce gains made during treatment. More intensive surveillance allows relapse to be quickly detected, and integrated protocols for addressing this could be developed proactively through partnerships between criminal justice and treatment system agencies. Others have indicated that a clinical profile for individuals most likely to benefit from legal pressure needs to be established. 63 references and 2 tables