Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder characterized by progressive loss of motor neurons. However, ALS has been recognized to also involve non-motor systems. Subclinical involvement of the autonomic system in ALS has been described. The recently developed 13 C-octanoic acid breath test allows the noninvasive measurement of gastric emptying. With this new technique we investigated 18 patients with ALS and 14 healthy volunteers. None of the patients had diabetes mellitus or other disorders known to cause autonomic dysfunction. The participants received a solid standard test meal labeled with 13 C-octanoic acid. Breath samples were taken at 15-min intervals for 5 h and were analyzed for 13 CO 2 by isotope selective nondispersive infrared spectrometry. Gastric emptying peak time (t peak) and emptying half time (t½) were determined. All healthy volunteers displayed normal gastric emptying with a mean emptying t½ of 138±34 (range 68–172) min. Gastric emptying was delayed (t½> 160 min) in 15 of 18 patients with ALS. Emptying t½ in ALS patients was 218±48 (range 126–278) min (p< 0.001). These results are compatible with autonomic involvement in patients with ALS, causing delayed gastric emptying of solids and encouraging the theory that ALS is a multisystem disease rather than a disease of the motor neurons only.