This study examines the association between physical and mental health conditions and disability days among a nationally representative working population, after controlling for basic sociodemographic variables, moderating variables, and comorbidities. Cross-sectional data from the Medical Expenditure Panel Survey (MEPS) for 2007 was used. T tests and linear regressions were used to examine differences in disability days by comorbidities. The average number of disability days by condition ranged from 4 days for impulse control disorders to a maximum of 18 days for stroke. Comorbidities explain all of the disability days for allergies. The contribution of comorbidities to disability days varied from 0% for stroke and 14% for cancer to 72% for diabetes and 77% for asthma. Among those with mental illnesses, comorbidities contributed 64% of the disability days for anxiety and only 18% for depression. The associations between comorbidities and disability days varied by type of physical and mental condition. Except for stroke, cancer, and depression, illness burden was exacerbated by comorbidities. Treatment strategies and plans must focus on comanagement of chronic conditions to reduce disability days.