Acute asthma exacerbations are one of the most common reasons for paediatric emergency department visits and hospitalisations, and a relapse frequently necessitates repeat urgent care. While care plans exist, there are no acute asthma prediction rules (APRs) to assess severity and predict outcome. The primary objective of the Acute Asthma Severity Assessment Protocol study is to develop a multivariable APR for acute asthma exacerbations in paediatric patients. A prospective, convenience sample of paediatric patients aged 5-17 years with acute asthma exacerbations who present to an urban, academic, tertiary paediatric emergency department was enrolled. The study protocol and data analysis plan conform to accepted biostatistical and clinical standards for clinical prediction rule development. Modelling of the APR will be performed once the entire sample size of 1500 has accrued. It is anticipated that the APR will improve resource utilisation in the emergency department, aid in standardisation of disease assessment and allow physician and non-physician providers to participate in earlier objective decision making. The objective of this report is to describe the study objectives and detailed methodology of the Acute Asthma Severity Assessment Protocol study.