Earlier results [1], suggesting an autocrine tumor cell stimulation by CSF-1, are in agreement with data by Fildermann et al. [2], showing an enhanced motility and invasiveness in the CSF-1 receptor expressing BT20 breast cancer cell line upon stimulation with recombinant CSF-1. Tumor-cell secreted CSF-1 has also been shown to cause monocyte recruitment, but not cytotoxicity [3]. Down-regulation of monocyte class II antigen expression after exposure to high concentrations of CSF-1 [4] may decrease macrophage-mediated tumor cytotoxicity and favor tolerance. Raised CSF-1 serum levels may thus increase tumor metastatic behavior as well as cause immune suppression in advanced stage disease. We set out to evaluate serum CSF-1 levels in primary and metastatic breast cancer. Serum samples from one hundred and eighteen primary breast cancer patients and seventy-five patients with metastatic disease were assayed by radio-immuno-assay (RIA) for circulating colony-stimulating factor 1. Mean serum levels were significantly higher in the metastatic population (9.7 ng/ml +/- 0.8) as compared to the patients with primary tumors (4.2 +/- 0.2) (p = 0.0001). Patients with early stage tumors (T0/T1/T2) had significantly lower levels than patients with tumors of larger size (T3/T4) (p = 0.0001). Relapse and survival statistics were analyzed using Kaplan-Meier estimates. Samples from 118 primary breast cancer patients were available to study. The median follow up was 85 months (range: 1-108). An elevated CSF-1 concentration (> 6.6 ng/ml or > 550 Units/ml) was associated with a shorter disease free interval (p = 0.03). In a multivariate analysis, including T (clinical tumor size), N (clinical node status), histological grade, and hormone receptor status, CSF-1 remained significantly associated with a poorer outcome (relative risk of relapse: RR: 3.3 [1.3-8.5]), together with tumor size (RR: 2.8[1-8.2]) and clinically involved nodes (RR: 4.1[2.1-8]). These results were not modified following adjustment for type of treatment. We conclude that raised circulating CSF-1 levels may be an indicator of early metastatic relapse.