Followup results from the Drug Abuse Treatment Outcome Studies (DATOS) 1-year and 5-year followups were used to describe the long-term outcomes of drug treatment and to further clarify the relationship between treatment duration and post-treatment outcomes in four treatment modalities: outpatient methadone, long-term residential (LTR), outpatient drug free (ODF), and short-term inpatient. Methods replicating those used in earlier analyses of the DATOS 1-year followup of 2,966 patients admitted to treatment in 1991-1993 and those of the Treatment Outcome Prospective Study patients admitted in 1979-1981 were employed. DATOS is a non-experimental longitudinal study conducted within the natural settings of 96 treatment programs in the U.S.A. The study followed patients during and after treatment at specified periods of time. Prevalence of drug use and behaviors were evaluated for the year prior to treatment; and the post-treatment time frames defined by the 1- and 5-year followups. In addition, the multivariate analytic technique of generalized estimating equations was used to examine the relationship of treatment duration and outcomes across both followups while also controlling for patient characteristics and pretreatment levels of behaviors. The 5-year stratified followup sample included 1,393 of the same individuals in the 1-year followup sample. Analyses were restricted to patients participating in both followups. Reductions in prevalence of cocaine use in the year after treatment (compared to the preadmission year) by patients were associated with longer treatment durations (particularly 6 months or more in LTR and ODF). In addition, reductions in illegal activity and increases in full-time employment were related to treatment stays of 6 months or longer for patients in LTR. The DATOS results from the 1-year and 5-year post-treatment followup combined suggest the stability of outcomes of substance abuse treatment. While results are generally consistent with the full 1-year followup, reduced sample size and bias of the sample toward patients with longer treatment retention may have attenuated the findings.