In recent years there has been an increased appreciation of the importance of measuring health status from the patient's point of view, but until now no attempt has been made to develop an amyotrophic lateral sclerosis (ALS)-specific health status measure. The development of such an instrument is especially relevant now with the introduction of drugs that prolong life in ALS but limited data is available on the impact such treatments have on quality of life. This paper reports on the development of an ALS-specific measure, the forty item ALS assessment questionnaire (ALSAQ-40). The development of the ALSAQ-40 followed three main stages. Stage 1 consisted of in-depth, semi-structured exploratory interviews conducted on a sample of 18 patients to identify areas of salience and concern to patients with ALS. These interviews generated 78 candidate questions. In stage 2, the 78-item questionnaire was used in a postal survey to identify appropriate rephrasing/shortening and to determine the acceptability of the measure. In addition, this exercise helped identify sub-scales of the instrument addressing different dimensions of ALS. Finally in stage 3 the data collected in stage 2 was analysed to areas measured by the instrument and to remove redundant questions. The resulting measure contains forty questions measuring five areas of health status: Eating and Drinking, Communication, ADL/independence, Physical mobility, Emotional Functioning. The measure has high face, internal and construct validity and is likely to prove a useful measure in the evaluation of treatment regimes for ALS/MND.