Public health researchers and practitioners have begun to recognize the dynamic nature of disability, promote the health of people with disabilities, and develop strategies to prevent secondary conditions among them. To understand the epidemiology of secondary conditions, the authors developed the Massachusetts Survey of Secondary Conditions, a longitudinal study of adults with major disabilities (n = 656) based on a conceptual framework linking disability, mediating factors, and health outcomes. This paper reports baseline data on the number of secondary conditions experienced by survey respondents. Respondents experienced a mean of 5.3 of 17 secondary conditions. More numerous secondary conditions were associated with fair or poor general health and number of days unable to do routine activities. Factors amenable to public health interventions included difficulty with weight and exercise maintenance, tobacco and marijuana use, and experiencing assault. Disability should be a focus in all public health research, policy, and programs.