Closed-loop and open-loop optokinetic nystagmus (OKN) eye movements were recorded with an electromagnetic scleral search coil in 16 patients with myasthenic ocular palsies, 8 patients with nonmyasthenic ocular palsies, and 4 normal controls. We calculated the summation of fast-phase amplitude per second (SFPAps), mean of fast-phase amplitude (MFPA) and mean of fast-phase peak velocity (MFPV) of OKN before and after administration of neostigmine. Under open-loop conditions, the myasthenic patients group demonstrated distinct increases in three parameters: SFPAps (P < 0.000001), MFPA (P < 0.001) and MFPV (P < 0.001) following neostigmine injection, and the increase in SFPAps was greater than those of MFPA and MFPV (P < 0.02), while the other two groups showed no significant increases. Under closed-loop condition, only five of the myasthenic patients showed significant changes. These results suggest that the change in SFPAps during open-loop OKN before and after neostigmine administration was a specific measurement that differentiated patients with myasthenic ocular palsies from the other groups.