A syndrome of paroxysmal, episodic sympathetic hyperactivity after acquired brain injury has been recognized for almost 60 years. This project sought to simplify the confused nomenclature for the condition (>31 eponyms) and simplify the nine overlapping sets of diagnostic criteria. A consensus-developed questionnaire based on a systematic review of the literature was circulated to a widely representative, international expert group utilizing a Delphi approach. Diagnostic criteria were dropped if group consensus failed to agree on their relative importance, with a goal of reaching a Cronbach α of 0.8 (suitable for research purposes). The resulting criteria were combined into an assessment measure for clinical and research settings. The consensus group recommend that the term "paroxysmal sympathetic hyperactivity" replace previous terms to describe the "syndrome, recognised in a subgroup of survivors of severe acquired brain injury, of simultaneous, paroxysmal transient increases in sympathetic [elevated heart rate, blood pressure, respiratory rate, temperature, sweating] and motor [posturing] activity." An 11 point probabilistic diagnostic scale was developed with reference to published criteria, yielding an acceptable Cronbach α of 0.8. These 11 items were proceduralized and combined with a symptom severity index to produce a diagnostic tool for use with adults (the paroxysmal sympathetic hyperactivity assessment measure [PSH-AM]). Development of a pediatric version of the scale and further research into the validity of the PSH-AM is recommended. The consensus position builds on previous literature to establish diagnostic definitions and criteria, an important move to standardize research and management of this condition.