Four patients with continuous muscle fibre activity (Isaacs-Mertens syndrome) are reported in this communication. Electrophysiological evidence of peripheral neuropathy was present in two patients. Impulse induced repetitive CMAPs were noted in two patients. In one case the repetitive activity increased upon proximal stimulation. Single fibre EMG recordings of the spontaneous activity revealed pathological discharges with variable frequencies. Each pathological discharge comprised of a primary discharge and 1-4 extradischarges. There was marked variability of the interval between the primary discharge and subsequent extradischarges.