Cystinosis is an autosomal recessive metabolic disorder in which nonprotein cystine accumulates within most body tissues due to a defect in lysosomal cystine transport. The pathognomonic manifestation of cystinosis is the presence of distinctive iridescent crystals within ocular tissues. Although these crystals have been detected within the extraocular muscles, no associated abnormality of eye movement has been described. We measured horizontal saccades of 5–30|Mo in six patients with infantile nephropathic cystinosis. Our data indicate that patients with cystinosis have slowed saccades. Saccadic duration, peak velocity, peak acceleration, and peak deceleration were all abnormal.