Objective: To compare the prevalence of fasciculation potentials (FPs) with F-responses between patients with amyotrophic lateral sclerosis (ALS) and patients with benign fasciculations. Methods: In seven patients with ALS and seven patients with benign fasciculations, high-density surface EMG was recorded for 15. min from the gastrocnemius muscle. Template matching was used to search for pairs of FPs with a repetition within 10-110. ms. Results: Interspike interval (ISI) histograms were constructed from 282 pairs of benign fasciculations and from 337 FP pairs in ALS. Peaks attributable to F-waves were found at latencies of 32. ms (benign) and 35. ms (ALS). Five patients with benign fasciculations and four patients with ALS had FPs with F-waves. Conclusions: F-waves of FPs occur in both conditions - therefore they are not diagnostically helpful. Significance: F-waves confirm the distal origin of FPs for an individual axon. The occurrence of these FPs in a benign condition suggests that the generation of ectopic discharges in the distal axons is not specific to progressive neurodegeneration. © 2011 International Federation of Clinical Neurophysiology.