Carcinomas of an unknown primary site (CUP) are heterogeneous tumours with a median survival of only 8 months. Tyrosine kinase inhibitors are promising new drugs. The aim of this study was to determine the expression of EGF-receptor, Her-2/neu, and c-Kit tyrosine kinases in CUP. Paraffin-embedded specimens were obtained from 54 patients with a CUP who were included in the GEFCAPI 01 randomised phase II trial. Immunohistochemistry was performed using the Dako autostainer with antibodies directed against HER-2/neu protein, EGFR protein, and c-Kit protein (CD117). EGFR expression was found in 36 out of 54 samples (66%). In contrast, Her-2/neu overexpression and c-Kit positivity were only detected in 4 and 10% of patients, respectively. No significant association was found between the expression of the tyrosine kinase receptors and prognosis. EGFR expression was significantly associated with response to cisplatin-based chemotherapy: the response rates were 50 and 22% in patients with EGFR-positive tumours and EGFR-negative tumours, respectively (P<0.05). This study shows that EGFR is frequently expressed in CUP. This finding may prompt clinical trials investigating EGFR inhibitors in this setting. In contrast, c-Kit expression and Her-2/neu overexpression occur infrequently in CUP. EGFR expression was correlated to tumour chemosensitivity.