The aim of this prospective, multicentre study was to investigate the effects of a false negative mammogram on treatment delay and tumour size. Among 306 consecutive women with histologically diagnosed, invasive breast cancer, the frequency of a false negative mammogram was small (13%) among women aged over 50 years, but 35% among those aged 50 or younger (P < 0.0001). Forty-five per cent of the women with a false negative mammogram had a longer than 2-month and 29% a longer than 6-month interval from mammography to surgery as compared with only 2 and 0% of women, respectively, who had a true positive mammogram (P < 0.0001 for both). Women with a false negative mammogram and a longer than 2-month interval to surgery had larger primary tumour size (60 versus 26% pT2-4, P = 0.005) and more often positive axillary nodes (60% versus 32% pN+, P = 0.03) at the time of surgery than those with a shorter delay. We conclude that a false negative mammogram is common in women younger than 50, and may lead to treatment delay and advanced clinical stage.