Factors affecting the adequacy of breast cancer screening rounds and the clinical implications of screening have been investigated in women aged 50-74 years. Questionnaires were sent to all eligible patients and to the family doctor of those patients who had died or were lost to follow-up at the time of the study. Unlike the tumour characteristics the patients' personal and family characteristics were also collected before the diagnosis. The clinical features of screening-detected tumours and those discovered on clinical signs or on metastasis were compared. On the basis of the time between two mammographies, we created the following four groups according to the recommended screening round: adequate, long, short and patients with no earlier mammography. Univariate and multivariate generalized logit models were obtained to determine factors affecting the adequacy of breast cancer screening rounds. Five hundred and thirty-three patients were included. Two hundred and seventy-seven (52%) had inadequate breast cancer screening rounds (long, short or no earlier mammography). The American Joint Committee on Cancer stage was less advanced (0/1) in screening-detected tumours and among tumours of patients with an adequate screening round (P=0.014). Multivariate analyses showed that patients with an earlier organized screening mammography (P<0.0001) and those with gynaecological follow-up (P=0.03) were more likely to have an adequate rather than an inadequate breast cancer screening round. Screening leads to the detection of early-stage tumours when it is performed according to the recommendations. Organizing mammography rounds as recommended is essential to optimize the benefits of breast cancer screening.