Acquired pendular nystagmus (APN) complicates multiple sclerosis and other neurological disorders, causes visual impairment, and frequently resists treatment. Vision could be improved by a visual aid that gates or shifts the seen world in lockstep with the APN. Since the pathological oscillations are embedded in normal eye movements, such a device must track the nystagmus selectively. We evaluated the ability of an adaptive filter to perform this tracking and improve acuity when coupled to either of two devices--a shutter that permitted brief glimpses of the world synchronized with the nystagmus, or simulated image-shifting optics. In 10 normal subjects whose decimal acuity averaged 1.46 +/- 0.20, acuity fell to 0.36 +/- 0.08 under viewing conditions simulating APN. The synchronized shutter restored acuity to 0.60 +/- 0.12, while image-stabilization raised it to 1.17 +/- 0.13. Adaptive filters provide a practical means by which to track nystagmus. The most effective visual aid would couple such filters to image-stabilizing optics.