Oscillopsia and nystagmus began in a woman 2 weeks after an inadvertent lumbar puncture during anesthesia for childbirth. Examination showed horizontal-torsional jerk nystagmus in all positions of gaze. Magnetic-search-coil oculography revealed accelerating slow phases, with an increase in nystagmus amplitude in darkness. Magnetic resonance images showed type 1 Arnold-Chiari malformation. Three months after occipital decompressive surgery, nystagmus had almost disappeared. Accelerating slow phases should not be considered diagnostic of congenital nystagmus, especially with an onset of oscillopsia in adult life; imaging should be considered to exclude treatable hindbrain anomalies. Lumbar puncture in patients with the Arnold-Chiari malformation may accentuate craniospinal pressure dissociation and precipitate neurological signs.