The aim of the study was to determine whether the Borg dyspnoea scale could be a useful and simple marker to predict respiratory muscle weakness in amyotrophic lateral sclerosis (ALS). From April 1997 to 2001, respiratory function was perfomed in 72 patients together with the Borg score in both the upright (uBorg) and supine (sBorg) positions. Mean upright vital capacity (VC) was 81+/-24% predicted, sniff nasal inspiratory pressure (SNIP) was 55+/-26% pred, maximal inspiratory pressure (P(I,max)) was 57+/-26% pred and arterial carbon dioxide tension (P(a,CO(2))) was 41+/-6 mmHg. The mean Borg scores in the upright and supine positions were 1.7+/-1.5 and 2.2+/-2, respectively. A significant relationship between SNIP and uBorg (r = 0.4; p = 0.0007) and SNIP and sBorg (r = 0.58; p<0.0001) was observed. Upright VC, DeltaVC (measured as the supine fall in VC as a percentage of seated VC), P(I,max) and P(a,CO(2)) were significantly correlated with SNIP. A cut-off value of 3 on the sBorg scale provided the best sensitivity (80%) and specificity (78%) (area under the curve 0.8) to predict a SNIP < or =40 cmH(2)O, indicating severe inspiratory muscle weakness. Patients with a sBorg score > or =3 also exhibited significantly lower VC, P(I,max) and twitch mouth pressure during cervical magnetic stimulation, and slightly higher P(a,CO(2)) (43.7+/-7 versus 39.2+/-5 mmHg; p = 0.05). The Borg dyspnoea scale is a valuable noninvasive test for the prediction of inspiratory muscle weakness in ALS patients.