The diagnosis of A-T is usually based on common clinical features (ataxia, telangiectasia, abnormal eye movement and speech) and supported by laboratory tests (see list at right). When all of the clinical signs and symptoms are observed (usually in older children and adults), the diagnosis is made relatively easily. In young children, the diagnosis is much harder to make. Often the only presenting symptom is ataxia. Telangiectasias do not usually occur until after the age of 5 years, and do not occur at all in some patients. Eye movements are almost always normal in young patients. A history of recurrent upper and lower respiratory tract infections may be another clue to the diagnosis.

One of the most important lab tests used in the diagnosis of A-T is the measurement of the alpha fetoprotein level in the blood, as about 95% of patients with A-T have elevated levels of alpha fetoprotein after age 18-24 months. Other confirmatory laboratory tests include:

Elevated level of the blood protein CA125
Increased cell death or chromosomal breakage after exposure of blood cells to x-rays in the laboratory
Absence of the ATM protein on a Western blot
Abnormal DNA sequence (mutation) of the
A-T gene (ATM)