A 5.5-year-old girl had myasthenic symptoms since birth. Tests for antiacetylcholine receptor (AChR) antibodies were negative. To investigate the character of the neuromuscular transmission defect, an intercostal muscle specimen was obtained at age 27 months. Immune deposits were absent from the endplates. On electron microscopy, most postsynaptic regions appeared normal, but the density of AChR on the junctional folds was diffusely reduced. In vitro microelectrode studies revealed that the number of transmitter quanta released by nerve impulse was normal. The amplitude of miniature of endplate potentials and currents was abnormally low. A study of the kinetic properties of AChR by analysis of acetylcholine-induced current noise demonstrated a significant decrease in mean channel open-time; the mean channel conductance was normal. The safety margin of neuromuscular transmission in this disorder is likely to be compromised by the deficiency and abnormal kinetic properties of AChR. The findings are unique among those patients with congenital AChR deficiency described to date.