National programmes of mammographic screening can lead to one in three breast cancers being overdiagnosed. Zahl and colleagues (p 921) report that the incidence of breast cancer among women 50-69 years of age increased by 50% when Norway and Sweden introduced mammographic screening. They also found no compensating fall in incidence over age 69, when women are no longer invited for screening. In the absence of screening, the authors say, one of three women in the 50-69 age group who were diagnosed with invasive breast cancer would not have been diagnosed as having breast cancer in her lifetime. The authors conclude that women cannot make an informed choice on screening unless the level of overdiagnosis is properly explained to them.