In this study, we evaluated the efficacy of bone marrow immunoscintigraphy (BMIS) for the detection of skeletal metastases in 23 patients with histologically confirmed breast cancer. All patients underwent whole-body BMIS 3-6 h after the intravenous injection of 0.20-0.33 mg of the intact anti-NCA 95 MAb BW 250/183 labelled with 259-555 MBq 99Tcm and a whole-body 99Tcm-MDP bone scan. In four patients, BMIS SPET of the lumbar spine was also performed. Serum alkaline phosphatase was determined in all patients and the level of human anti-mouse antibody (HAMA) in 16. Final diagnosis was confirmed by radiology and 2 years follow-up. Compared with the 99Tcm-MDP bone scan, BMIS demonstrated better specificity (88% vs 75%) and a better positive predictive value (92% vs 85%). There were no significant differences between BMIS and the bone scan in the detection of skeletal metastases (P > 0.05). In one patient with normal planar BMIS of the lumbar spine, SPET disclosed a metastatic lesion in the bone marrow. The correlation coefficient between BMIS and bone scan and between BMIS and serum alkaline phosphatase was r = 0.688 and r = 0.483 respectively. One patient developed a minor HAMA response after BMIS. Patients with diffuse increased activity of the skull on the bone scan had a significantly higher skull to whole body ratio on BMIS (P < 0.01). Thus BMIS can improve the specificity and positive predictive value of bone scanning in the detection of skeletal metastases, with a low HAMA response. Diffuse increased activity of the skull on bone scans could be explained by bone marrow extension. SPET scanning of the spine may improve the sensitivity of BMIS.