Amyotrophic lateral sclerosis (ALS) is a progressive disorder of unknown origin. Respiratory involvement is the principal cause of death, and dyspnoea is a major source of discomfort. In this study, diaphragm function is described and its relationship with dyspnoea examined in 48 ALS patients (32 male, age 26-80 yrs). The detailed neurological and respiratory evaluation (clinical examination, pulmonary function tests, static pressures, mouth twitch pressures (Pm,t), electromyographic responses to phrenic nerve stimulation and cortical magnetic stimulation were analysed after stratification according to dyspnoea. Dyspnoeic (group I) and nondyspnoeic (group II) patients were similar, bulbar signs being more frequent in group I. Vital capacity was lower in group I (mean+/-SD 67.9+/-22.7 versus 87.9+/-15.6% of the predicted value, p=0.0028), as were maximal static inspiratory pressure (41+/-24 versus 60+/-27% pred, p=0.0242) maximal static inspiratory pressure (18+/-11 versus 32+/-14% pred, p=0.0042), and Pm,t (3.71+/-2.5 versus 7.26+/-3.45 cmH2O, p=0.0011). Abdominal (Abd) paradox and respiratory pulse were frequent in group I (15 of 25 and 14 of 25) but absent or rare in group II (0 of 23 and four of 23) (p<0.05). The electromyographic responses to phrenic and cortical stimulation were generally abnormal in group I but subnormal in group II. Multivariate analysis selected only signs of diaphragm dysfunction (namely, Abd paradox and abnormal electromyographic responses) as significant predictors of dyspnoea. It is concluded that dyspnoea in amyotrophic lateral sclerosis patients should prompt diaphragm function tests.