Preliminary bone scans have been performed on all patients with early breast cancer. The incidence of positive scans has been recorded and several factors determining scan status have been enumerated. Particular attention has been focused on the natrual history of scan positive and scan negative patients. Eighteen per cent of stage I and 41 per cent of stage II cancers had positive bone scans. Scan results were correlated with age, menopausal status, tumour position, tumour size and histological node status. Postmenopausal patients were found to have a significantly increased risk of being scan positive (P less than 0-01). Follow-up studies have confirmed that the lesions demonstrated by scanning actually represent metastatic foci. At 18 months 85-7 per cent of scan positive patients had evidence of disseminated disease compared with only 11-4 per cent of scan negative patients (P less than 0-01). Clinically overt advanced disease evolves from positive scan lesions. It is clear that a significant percentage of patients felt to have early breast cancer already have widely disseminated disease at the initial presentation. The biological significance of bone scan lesions makes a sensitive screening test for dissemination an essential part of the preliminary assessment of patients with breast cancer. Bone scans provide an excellent prognostic index at a patient's initial assessment.