We present an economic argument for the effect of employment on length of stay in substance abuse treatment, which is documented to contribute to positive post-treatment outcomes. We begin by presenting a theoretical model which predicts longer lengths of stay for employed versus unemployed individuals, as is observed empirically. The model shows that along with its psychological benefits, employment increases length of stay by increasing the opportunity costs of substance use. As labor market outcomes and experiences are different between men and women we empirically examine the gender specific effects of employment on length of stay. Furthermore, we consider that there are different types of employment. We conduct an econometric analysis of the Treatment and Episode Data Set for discharges (TEDS-D) examining the effect of being full time, part time and unemployed. The results agree with previous research and the economic model presented, any employment increases length of stay. However, men receive greater benefits from both types of employment. Also, men receive the greatest increase from full time employment, whereas women from part time employment. The results further suggest the need for gender specific treatment policies.