Length of stay in treatment has been found to be a significant predictor of positive post-treatment outcomes, such as decreases in unemployment and crime. However, length of stay may be an incomplete predictor of successful treatment. Surprisingly, few studies have examined whether completing treatment in addition to length of stay is an important factor in explaining positive treatment outcomes. The objective of our study is to examine the effect that treatment completion and length of stay have on post-treatment employment and crime for patients in outpatient drug-free treatment, the largest treatment modality in the United States. We use conditional logit and multiple regression models with program-level indicator variables (fixed effects) to estimate the effect of treatment completion and length of stay on employment and crime controlling for drug use severity, previous treatment history, and other patient demographics. Data are from the National Treatment Improvement Evaluation Study and include 986 adults enrolled in outpatient drug-free programs across the United States. We find that treatment completion and length of stay are significantly related to post-treatment employment. Holding length of stay constant, the occurrence of employment at follow-up among patients who complete their planned treatment is almost 2 times that of patients who do not complete treatment. However, treatment completion did not have a statistically significant effect on the probability of post-treatment crime. Although our results are mixed, these findings suggest that greater attention should be placed on evaluating the importance of both length of stay and treatment completion in treatment outcome studies.