Medical investigations use a wide variety of outcome indicators that are often not comparable. It can be challenging to integrate results across multiple studies that do not share a common metric. Some conditions such as Duchenne and Becker muscular dystrophy have a predictable course of disease progression. Severity can be inferred from a patient's medical history. This paper describes the development of a disease severity measure using common markers of disease progression. Rasch modeling was used to estimate severity using dichotomous events that indicate disease progression. Caregivers of 34 young men with Duchenne or Becker muscular dystrophy completed structured interviews about their care and medical history. Interview questions included surgeries (tendon release, scoliosis, tracheostomy), respiratory equipment (assisted ventilation, cough assist devices), and the use of other medical equipment (e.g., braces, walkers, wheelchairs, transfer boards, hospital beds). The resulting measure had a reliability of .83. The correlation between the severity measure and the Amyotrophic Lateral Sclerosis Functional Rating Scale (ALSFRS) was .68. Preliminary results and item calibrations are provided for the severity measure that can be estimated from caregiver reports or administrative data.