Eighty-four patients with breast cancer at high risk of bone metastases were investigated with magnetic resonance imaging (MRI) of the thoracolumbar spine. Of 58 patients with normal limited skeletal surveys (LSS) and bone scans (BS), 4 (7%) had MR images compatible with malignant infiltration. Fourteen patients had abnormal bone scans with normal or non-diagnostic plain films; 7 of these patients (50%) had MR images compatible with malignant infiltration. Twelve patients had single or multiple wedge collapses of uncertain aetiology on plain film; MR demonstrated metastatic disease as the cause of wedge collapse in 7 (58%). MRI may define a group of patients with extra-osseous relapse who have occult metastatic disease. Although the detection rate in patients with primary breast cancer is low (4/45), MRI is of value in determining the cause of wedge collapse in postmenopausal women with breast cancer and may elucidate the cause of an abnormal bone scan with normal or non-diagnostic plain films.