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Current mammography guidelines reflect evidence that mammography could be harmful on average through the overdiagnosis of breast cancers that would not eventually cause symptoms in the long term. To inform targeting within these guidelines, I investigate whether some women are more likely to experience overdiagnosis than others on the basis of their mammography behavior. Using data on mammography behavior within an influential clinical trial, random assignment, and a model, I proceed in two steps. First, I find that women who are more likely to receive mammograms are healthier and have higher socioeconomic status. Second, building on the first finding, I find that the 20-year level of overdiagnosis is at least 3.5 times higher among women who are more likely to receive mammograms, such that at least 36% of their cancers are overdiagnosed. Current guidelines presuppose that the women most likely to receive mammograms are the women most likely to benefit from them. My findings imply that these guidelines could have unintended consequences by effectively encouraging mammograms among healthier women who are more likely to be overdiagnosed by them.