Basic fibroblast growth factor (bFGF) is a potent endothelial cell mitogen found in a variety of normal and tumour tissues and is of prognostic relevance in human malignancies such as renal cell carcinoma and leukaemia. This study presents the data of 104 serum samples of 20 patients suffering from breast cancer. Mean serum levels of bFGF in these patients were 13.9 +/- 17. 1 (min 0, max 56.4) pg/ml and 2.4 +/- 5.9 (min 0, max 24.7) pg/ml, respectively (p = 0.01). Basic FGF reached a sensitivity of 61% at a specificity of 87% when applying a cut-off level of 5 pg/ml. A continuous increase of bFGF serum levels before the clinical detection of relapse (lead time) was seen in 3 out of 8 cases with a mean lead time of 4 months. Preoperative serum levels were not of prognostic value and showed no correlation with axillary lymph node metastasis. These preliminary results indicate that, in breast cancer patients, soluble bFGF may be useful in early detection of primary tumours, recurrences and monitoring of therapy.