Mutations have been described in the ataxia telangiectasia mutated (ATM) gene in small numbers of cases of lymphoid neoplasia. However, surveys of the ATM mutation status in lymphoma have been limited due to the large size (62 exons) and complex mutational spectrum of this gene. We have used microarray-based assays with 250,000 oligonucleotides to screen lymphomas from 120 patients for all possible ATM coding and splice junction mutations. The subtypes included were diffuse large B cell, mantle cell, immunoblastic large B cell, follicular, posttransplant lymphoproliferative disorder, and peripheral T cell lymphoma. We found the highest percentage of ATM mutations within the mantle cell (MCL) subtype (43%, 12 of 28 cases), followed by a lower level (10% of cases) in the other subtypes. A frame-shift ATM mutation was found in one peripheral T cell lymphoma patient. In six MCL cases examined, four ATM variants were due to somatic mutation in the tumor cells whereas two others seemed to be germ-line in origin. There was no difference in p53 mutation status in the ATM mutant and wild-type groups of MCL. There was no statistically significant difference in the median overall survival of patients with wild-type vs. mutated ATM in MCL. Additional mutational and functional analyses are needed to determine whether ATM mutations contribute to the development and progression of MCL or are just the consequence of genomic instability in MCL.