Seizure severity scales have recently been identified as an important additional outcome measure in trials of new antiepileptic drugs (AEDs). The National Hospital Seizure Severity Scale (NHS3) is presented as a refined version of the Chalfont Seizure Severity Scale. The principal advantages of the new version are that it is quicker and simpler to apply, the limits of reliability are now clearly defined, and construct validity for the scale is available. The scale is administered by a health professional during an interview with a patient and a witness to the seizures. It contains seven seizure-related factors and generates a score from 1 to 27. An intraclass correlation coefficient of 0.90 was obtained during interobserver and test-retest reliability assessment, suggesting that the scale is sufficiently reliable for group studies. Scores for an individual patient should be interpreted with caution in light of the limits of agreement obtained. Validation experiments indicate that NHS3 measures seizure severity in a manner compatible with the subjective impression of people with epilepsy. We suggest that the NHS3 is a valid, easily applicable measure of seizure severity that is acceptably reliable for use in trials of novel AEDs.