Tumour expression of the macrophage colony stimulating factor (CSF-1 or MCSF) has been associated with an adverse prognosis in breast cancer, through an effect on the promotion of metastasis. The aim of the present study was to evaluate the clinical relevance of high circulating CSF-1 levels in patients with newly diagnosed breast tumours and correlate CSF-1 with clinico-pathological parameters. A secondary aim was to also measure CSF-1 in patients with other tumour types and at different stages of disease. Using a commercially available ELISA, pre-treatment plasma levels of CSF-1 were assessed, in 471 consecutive patients diagnosed with breast tumours, in 70 patients with newly diagnosed cancer of the head & neck, in 32 men with prostate cancer metastatic to bone and in 39 women with advanced metastatic breast cancer. Mean CSF-1 levels were significantly higher in patients with locally advanced (p <.015) or metastatic breast tumours (p <.048) and in a group of primary breast cancer patients (n = 26) selected for intensive chemotherapy because of multiple adverse tumour characteristics (p <.0002). Mean CSF-1 was also higher in patients younger than 35 years (p <.02) and in post-menopausal patients (p <.03). There was no significant association with tumour histologic type, grade, or other individual histopathologic parameters. No significant association was found between pre-treatment CSF-1 and overall/relapse free survival. Median CSF-1 levels were dramatically higher in patients with newly diagnosed tumours of the head & neck (604 pg/ml), in men with prostate cancer metastatic to bone (627 pg/ml) and women with advanced metastatic breast cancer (867 pg/ml) than those seen in patients with newly diagnosed breast tumours (334 pg/ml). Our data support the hypothesis that CSF-1 may play a functional role in tumour progression to metastasis as has previously been reported in animal models.