We describe two unrelated patients with Gillespie syndrome (partial aniridia, cerebellar ataxia, and mental retardation). The typical presentation is the discovery of fixed dilated pupils in a hypotonic infant. The iris abnormality is specific and seems pathognomonic of Gillespie syndrome. It can be distinguished clinically from other forms of aniridia and a presumptive diagnosis of Gillespie syndrome can be made in the first months of life on the basis of the ocular findings. Neurological involvement includes marked motor delay, hypotonia, disabling ataxia, and usually mental retardation. Cerebral and cerebellar atrophy with white matter changes on MRI scan were present in our second patient suggesting that patients with Gillespie syndrome may have more extensive CNS involvement than previously described. The parents of this child were first cousins; thus Gillespie syndrome may be heterogeneous with autosomal recessive and autosomal dominant forms.