We have carried out an audit of breast screening by mammography under 50 years of age in a cohort of 192 women attending family cancer clinics run by the South Thames genetic services. Of these women, six came from families in which a BRCA mutation had been identified, 61 had > 50%, 35 a 20-50% and 90 had < 20% chance of carrying a high risk mutation. In the 192 women in the screened cohort, 9 breast cancers were diagnosed (4.7%), all in high-risk women. Three were diagnosed at the prevalence screen. Three were detected mammographically at subsequent screening rounds; three were detected by breast self-examination (BSE) between screening episodes. One interval cancer was visible on mammogram at presentation but not at screening five months previously. A second cancer was also visible on mammogram at presentation but the normal screening mammogram had been 17 months earlier, outside the recommended interval. The remaining interval cancer was not visible on the mammogram. A total of 363 two-view screening mammograms were performed in the 280 person-years of follow-up; 109 additional investigations were generated: 23 recall mammograms, 18 symptomatic mammograms, 45 ultrasounds, 12 aspiration cytologies and 11 biopsies. Cytology diagnosed malignancy in 1 of 12 cases; breast biopsy in 9 of 11 cases. Twenty-three additional women had ultrasound screening only. This audit suggests that screening below the age 50 years may be unnecessary in families with a low chance of having a BRCA1 or -2 mutation, but it is important to screen high-risk women at least annually and possibly under 35 years.