OBJECTIVE: To assess the risks of induction of breast tumours from frequent screening mammography in younger females. METHODS: A study group of 853 females was identified who had at least 5 mammograms starting before 37 years of age, with 4 or more before the age of 40 years. These were followed up from their 40th birthday or 8 years from their first mammogram, and their cancer incidence was compared with that of a control group of 1103 females who had an average of 5 mammograms between the ages of 40 and 46 years. All females in the study were previously assessed to be at moderate familial risk or higher. RESULTS: There were 43 incident breast cancers in the study group after the 8-year start point, whereas 38.3 were expected from life-table calculations (RR 1.12; 95% CI: 0.83 to 1.51). In the control group, 50 incident breast cancers developed some time after their first mammogram in follow up to age 60 years. The observed, expected ratio from life tables in this group was 0.94 (95% CI: 0.71–1.24), similar to that in the study group. CONCLUSION: There was no trend to greater cancer incidence in those receiving mammograms earlier. ADVANCES IN KNOWLEDGE: This study shows that there is no substantial effect on the induction of additional primary breast tumours from frequent mammography starting at <37 years of age. Further work on larger numbers of females is necessary to assess longer term risks and determine whether a small excess cancer effect may be present.