Psychotropic medications are a common treatment for mental illness in people with developmental disabilities. Medication adherence is a critical determinant of the effectiveness of psychotropic drugs, but psychotropic medication adherence research specific to this population remains limited. This retrospective study analyzed Marketscan® Multi-State Medicaid Database 2003-2007 data. A total of 3,905 eligible community-based Medicaid enrollees aged 18-64 with dual diagnosis of developmental disability and mental illness were included and followed over an average of 510 days. Medication adherence was measured by Proportion of Days Covered (PDC), and logistic regression was used to assess factors associated with adherence. The mean PDC was 0.86, and the non-adherence rate was 25.7%. Factors associated with poor psychotropic medication adherence included minority race/ethnicity, female gender, enrollment in a Medicaid capitated plan, and having gastroesophageal reflux disease (GERD). Better psychotropic medication adherence was related to reductions in emergency room visits, suggesting better utilization outcomes.