11036 Background: Truncating and missense mutations in the ATM gene, causing insufficient DNA damage surveillance, allow damaged cells to proceed into mitosis, eventually resulting in increased cancer susceptibility. We tested the hypotheses that ATM Ser49Cys and ATM Ser707Pro heterozygosity increase the risk of cancer overall, of breast cancer, and of 26 other cancer subtypes in the general population.,We genotyped 10,394 individuals from the Danish general population followed for 36 years, during which 2056 developed cancer.,Multifactorially adjusted hazard ratios for ATM Ser49Cys heterozygotes versus non-carriers were 1.2 (95% CI: 0.9-1.5) for cancer overall, 0.8 (0.3-2.0) for breast cancer, 4.8 (2.2-11) for melanoma, 2.3 (1.1-5.0) for prostate cancer, and 3.4 (1.1-11) for cancer of the oral cavity/pharynx. Multifactorially adjusted hazard ratios for ATM Ser707Pro heterozygotes versus non-carriers were 0.8 (0.6-1.2) for cancer overall, 0.6 (0.2-1.6) for breast cancer, 10 (1.1-93) for thyroid/other endocrine tumours, and 2.7 (1.0-7.6) for cancer of corpus uteri.,ATM missense mutations do not increase risk of cancer overall or of breast cancer in the general population; however, we observed that ATM missense mutations associate with increased risk of melanoma, prostate cancer, cancer of the oral cavity/pharynx, thyroid/other endocrine tumours, and cancer of corpus uteri. No significant financial relationships to disclose.