Readmissions are costly, potentially harmful, and often avoidable. Medicare has begun to penalize hospitals for having higher-than-expected rates of AMI, HF, and Pneumonia; all chronic conditions, but other conditions like mental health disorders may also be contributing to larger numbers of readmissions in San Diego County. This study aims to investigate the burden attributable from mental health and substance abuse conditions upon San Diego County readmissions. A poisson regression analysis was used to compare the difference in numbers of readmissions for those with mental health or substance abuse diagnosis as their primary diagnosis compared to the primary three chronic diagnoses deemed important according to Medicare in the total San Diego County readmission population and in the Medicare readmission population in San Diego County. Patients with a mental health primary diagnosis were more likely to have a higher number of readmissions compared to patients with the top three chronic conditions in the total San Diego County population (IRR 1.172, p-val <.0001). Mental health was not statistically significant in the Medicare beneficiary population. Those patients with substance abuse as their primary diagnosis were less likely to have a higher number of readmissions in both populations. As efforts in San Diego County are made to find solutions to the problem readmissions pose, identifying whether or not mental illnesses play a critical role is necessary for the development of preventative programs targeting readmissions.