OBJECTIVE Despite the success of docetaxel as an anti-tumour agent, the inter-individual variability in drug response still poses a major impediment to further use of this agent in the treatment of cancer. Current knowledge about predictive biomarkers of docetaxel sensitivity in malignant effusions is poor. The aim of this study was to investigate the association between beta-tubulin III mRNA expression and chemosensitivity to docetaxel in metastatic malignant effusions. METHODS Real-time quantitative PCR was used for analysis of beta-tubulin III mRNA expression in 37 malignant effusions collected prospectively. Viable tumour cells obtained from malignant effusions were tested for sensitivity to docetaxel using ATP-TCA assay. RESULTS beta-tubulin III expression was inversely correlated with sensitivity to docetaxel in pleural effusions of NSCLC patients (P =0.022). The lower level of beta-tubulin III mRNA expression in malignant effusions was associated with higher chemosensitivity to docetaxel in NSCLC patients in vitro. No correlation was found between beta-tubulin III mRNA expression and docetaxel sensitivity in malignant effusions of gastric cancer patient. CONCLUSIONS Our results demonstrated that beta-tubulin III mRNA expression level in malignant effusions, in which all cancer cells were metastatic, was correlated with docetaxel sensitivity in NSCLC. This highlights the potential role of biomarkers in malignant effusions in further customized chemotherapy.