The incidence and characteristics of nystagmus in Down's syndrome are unclear. In 188 consecutive patients, 56 had nystagmus. Most had no clinically recognizable ocular pathology to account for the nystagmus. Twenty-nine had fine rapid horizontal nystagmus, 14 had a dissociated nystagmus which appeared pendular, whereas 9 had a form of latent or manifest latent nystagmus. Of the total patients with nystagmus, 41 had esotropia. Our findings suggest that fine rapid horizontal nystagmus, sometimes dissociated, occurs frequently in patients with Down's syndrome.