OBJECTIVE To evaluate the value of magnetic resonance imaging (MRI) in detecting bone marrow metastases in patients with breast cancer. METHODS Twenty-three patients with breast cancer in various stages (stage IV, 11; stage III, five; stage II, seven) were evaluated for bone marrow involvement. MRIs of marrow from lumbar spine, pelvis, and proximal femora were obtained with a 1.5-Tesla unit. All patients underwent bilateral bone marrow aspirations and biopsies for histologic evaluation and immunostaining with monoclonal antibody (MoAB) against low-molecular weight cytokeratin (CAM 5.2). Marrow MRI findings were compared with technetium 99m bone scans. Patients with stage II or III disease were monitored for clinical outcome. Possible correlation of MRI findings with serum alkaline phosphatase level was explored. RESULTS Fourteen of 23 patients showed MRI abnormalities suggestive of metastatic marrow disease (stage IV, nine; stage III, two; stage II, three). In six patients with abnormal MRIs, histology and MoAB immunostaining confirmed marrow involvement (stage IV, five; stage III, zero; stage II, one). In the other eight patients with MRI abnormalities, neither of these methods confirmed the presence of marrow metastasis. Four of five operable breast cancer (stage II-III) patients with an abnormal initial MRI showed additional abnormalities on follow-up examination and developed metastatic disease within 5 to 18 months demonstrable by conventional clinical methods. Conversely, none of the operable patients with negative MRIs developed recurrent disease at 3 to 16 months (Student's t test, P = .01). Nine patients with a normal MRI had no evidence of marrow involvement with histologic or MoAB immunostaining (stage IV, two; stage III, two; stage II, five). Of 14 patients with abnormal MRIs, bone scans were normal in seven and failed to show corresponding abnormalities in six. Elevated serum alkaline phosphatase levels showed a direct relationship with abnormal bone scans indicating extensive bony involvement, but failed to correlate with positive marrow MRIs. CONCLUSIONS MRI is a promising new technique to detect occult marrow involvement in breast cancer patients. There is a good correlation between abnormal marrow MRI and early development of clinical metastatic disease in patients with stage II to III disease.