To our knowledge, this is the third (possibly the fourth) description of a family with partial aplasia of the cerebellar vermis. The major clinical features are normal intelligence, delayed achievement of motor milestones, truncal ataxia, and nystagmus. Twelve of 14 affected individuals are female. The two affected males were more severely affected than were their female relatives. These findings along with apparently increased fetal wastage suggest that an X-linked rather than an autosomal dominant mode of inheritance may be responsible for this disorder.