Ataxia-oculomotor apraxia is a distinct entity first comprehensively described in 1988. The features include early childhood onset of ataxia and oculomotor apraxia, mimicking ataxia telangiectasia but without the extraneurologic findings of ataxia telangiectasia. We add to the clinical description of the ataxia-oculomotor apraxia syndrome by reporting eight patients, ages 2 to 15 years, from four families, suggesting autosomal recessive inheritance, with the longest follow-up over 6 years. After initial gait deterioration, all had a nonprogressive course. We have postulated that ataxia-oculomotor apraxia should be established as a separate disease from ataxia telangiectasia or its variants not only by clinical history, examination findings, and course, but primarily by the biologic markers of normal chromosome breakage and radiation sensitivity studies. We found no increased chromosome breakage in the four patients studied and intermediate sensitivity to chronic ionizing radiation of cultured skin fibroblasts on the three patients studied. Family studies revealed an intermediate radiosensitivity from two patients, their asymptomatic parents, and a sister. The lack of chromosome breakage strongly separates ataxia-oculomotor apraxia from ataxia telangiectasia. The radiation sensitivity studies are compatible with two possibilities: (1) symptomatic ataxia telangiectasia heterozygotes, but this would be highly unusual because the degree of clinical involvement in the ataxia-oculomotor apraxia patients is not mild, as would be expected if they were heterozygotes and (2) a separable disease entity, which is the interpretation we favor.