Inherited biallelic mutations of the ATM (ataxia-telangiectasia mutated) gene cause ataxia-telangiectasia, a rare autosomal recessive disorder associated with a high incidence of childhood leukaemias and lymphomas, suggesting that ATM gene alterations may be involved in lymphomagenesis. Loss of heterozygosity at 11q22-23 (location of the ATM gene) is a frequent event in sporadic lymphoid tumours, and several studies have reported a high prevalence of ATM gene alterations in diverse sporadic lymphoproliferative disorders, adding evidence to the postulated contribution of ATM in the pathogenesis of these tumours. This mini-review will summarize the recently published data concerning the ATM gene in sporadic lymphoid malignancies and will discuss the apparent paradox between the predominance of nonsense mutations observed in patient with ataxia-telangiectasia and the high proportion of missense alterations found in sporadic lymphoid tumours.