Mutations in genes encoding ion channel subunits have been described in association with a growing number of central and peripheral nervous system phenotypes. The Kv3 subfamily of potassium channels are encoded by four genes, KCNC1-KCNC4. They play an important role in the high-frequency firing of action potentials in the brain. Here we describe a family in which the three affected members exhibit various degrees of cognitive delay, ataxia and seizures. Cytogenetic analysis identified a deletion on chromosome 12q of the Shaw-related potassium channel gene, KCNC2 which encodes Kv3.2. We present the clinical, genetic and electrophysiological findings on this family in whom a new potassium channel defect has been identified. In addition to implicating important roles for potassium channels in neurodevelopment, these findings expand the clinical spectrum of potassium channelopathies.