Ataxia-telangiectasia (AT) is a rare multisystem, neurodegenerative genetic disorder that is characterised by progressive neurological abnormalities, oculocutaneous telangiectasias and immunodeficiency. Delay in diagnosis or misdiagnosis is probable due to its wide clinical heterogeneity in infancy. Recurrent sinopulmonary infections are often the only presenting symptom and usually patients have decreased immunoglobulins. A total 10% of patients who present with decreased serum immunoglobulin G and A and with normal or elevated immunoglobulin M levels are often misdiagnosed as hyperimmunoglobulin M syndrome. Definitive diagnosis is made if a patient with progressive cerebellar ataxia has a disease causing mutation on the ATM gene. Ataxia-telangiectasia guideline of the European Society for Immunodeficiencies defines the probable diagnosis criteria. We evaluated twenty ataxia-telangiectasia patients (mean age 13.8 +/- 4.1 years) retrospectively who were followed-up for a mean of 38.6 +/- 27.0 months. Twelve patients had a family history of consanguinity. A total of 80% patients suffered from various infections. Neoplasms occurred in three of them. Patients showed immunological abnormalities as low IgG (45%), low IgA (65%) and elevated IgM (60%) levels. CD3(+)CD4(+) T lymphocyte frequency was low in 45% patients. The mean AFP concentration at the diagnosis was 191.9 +/- 140.1 ng/mL and the raised IgM values did not show any statistically significant relationship with high AFP concentrations. Frequency of the elevated IgM concentrations in (60%) patients raises the concerns about thinking this finding has to be accepted as a probable diagnosis criterium.