Ataxia telangiectasia patients, with constitutional bi-allelic ATM mutations, have a marked risk of lymphoid tumors and ATM mutation carriers have a smaller risk of cancer. Sporadic ATM mutations occur in 10-20% of chronic lymphocytic leukemia and are often associated with chromosome 11q deletions which cause loss of an ATM allele. The role of constitutional ATM mutations in the pathogenesis of chronic lymphocytic leukemia is unknown. Here we investigated the frequency of constitutional ATM mutations in either of two chronic lymphocytic leukemia cohorts, those with and without a chromosome 11q deletion. We found that in comparison to controls, constitutional pathogenic ATM mutations were increased in patients with chromosome 11q deletions (6 of 140 vs. 0 of 281, P = 0.001) but not in those without 11q deletions (2 of 178 vs. 0 of 281, P = 0.15). These results suggest that ATM germline heterozygosity does not play a role in chronic lymphocytic leukemia initiation but rather influences rapid disease progression through ATM loss.