There has been much attention given to drugs and incarceration since the "War on Drugs" instituted by the Nixon administration, and the population of US prisons has been growing steadily due to individuals incarcerated for drug offenses. In response to increased incarceration, much attention has been given to rehabilitation of offenders, in particular drug offenders. Not much is known, however, about how gender, ethnicity and mental health affect participants' ability to engage in treatment while incarcerated. Previously, those varibles have been treated as confounds, yet they have proved to be of significance in prior research. The effects of gender, ethnicity and mental health on treatment engagement are examined in this study. Archival data were obtained from a two year study involving male and female inmates incarcerated in California Department of Corrections and Rehabilitations. Logistic regression and chi-square test of association were used to examine the relationship between gender, ethnicity, mental health, drug of choice, treatment engagement and treatment completion. Results suggest that African American and Latino participants were less likely to remain in treatment over 90 days, and to drop out of treatment before completion than their Caucasian counterpart. Results also suggest that female participants were less likely to remain in treatment over 90 days, and to drop out of treatment before completion than their male counterpart. In addition, it was found that participants with a mood disorder, but not a psychotic disorder, were more likely to complete treatment. Results and supplemental analysis point out to a possible bias in the incarceration rates, based on drug of choice, as well as possible culturally or gender biased treatment. Further research is recommended to expand on these ideas, as it is important to understand treatment efficacy across gender, culture, and mental health.