In amyotrophic lateral sclerosis (ALS), muscles with preserved strength can have fasciculation potentials (FPs) as the only abnormality. The FPs in strong muscles are predominantly simple and stable, and can often be recruited by a slight voluntary contraction. In weak and atrophic muscles, the FPs are generally complex and unstable, tend to have a slower firing rate and are not recruited by voluntary contraction. Macro-electromyography studies suggest that these FPs are part of a more complex motor unit, as opposed to simple FPs which can represent all the motor unit. FPs driven by transcranial magnetic stimulation (TMS) were observed in 13 ALS patients. TMS-driven fasciculations had a simple morphology and were stable. Complex potentials were never cortically driven. These observations strengthen the conviction that fasciculations in ALS can have different origins. Simple, stable FPs arise proximally and are probably related to excitotoxicity phenomena, while complex, unstable FPs are most likely to originate in distal axonal sprouts that are associated with the reinnervation process. Some ALS patients with fatigue and mild weakness can have profuse fasciculations at an early phase in the evolution of the disease. Patients with denervation localized in one region and diffuse FPs should be strongly suspected of having ALS. With this strategy it should be possible to shorten the diagnosis time.