The development of a rating scale for neuroleptic malignant syndrome (NMS) is described. The clinical and laboratory features of NMS were categorised into six domains after a thorough literature review and examination of patients. The reliability of this scale was established on 25 NMS patients and 50 control subjects based on chart reviews. A factor analysis supported a six-factor solution. The validity of the scale was indicated by the relationship of the severity rating to duration of illness and outcome. The inter-rater reliability of the scale was established prospectively in 10 subjects. The scale offers a measure of severity of NMS in the clinical setting so as to support the clinical diagnosis, monitor patients and determine their progress. The scale may be applicable not only to NMS or suspected NMS but also to NMS-like syndromes such as lethal catatonia.