The aim of the current pilot study was to determine whether placental isoferritin (PLF) can be detected in the serum of patients with metastatic breast cancer. Sera were obtained from breast cancer patients with metastatic disease (n = 100), from breast cancer with no evidence of disease (n = 70) and from healthy female controls (n = 34). PLF and total serum ferritin levels were independently measured using specific monoclonal antibody ELISAs in a double-blind study. It was found that the mean serum PLF levels were significantly elevated only in patients with visceral metastases (lung, liver, brain) compared with the levels of patients with non-visceral metastases (bone, skin) or with healthy controls. Contrary to this, analysis of total serum ferritin levels did not reveal significant differences between these groups. Considering 0-10 units/ml as a PLF negative result, it was found that PLF was negative in 87.5% of healthy controls and in 96% of breast cancer patients with no evidence of disease. In contrast, PLF was positive in 73% of the patients with visceral metastases and in 29.7% of those with non-visceral metastases. The striking difference between visceral and non-visceral metastases is not yet understood. It could result from a difference in the degree of vascularisation or, alternatively, a difference in the cell types and genes expressed by cells metastasizing to visceral or non-visceral organs.