It has been shown that hypoxia can induce resistance to a number of antineoplastic agents. Since vessel density may be considered as an indirect measure of the oxygenation of tumours, in this study we analysed the relationship between tumour vascularity or vascular endothelial growth factor (VEGF) expression and drug resistance. Tumour specimens of 152 non-small cell lung carcinomas (NSCLC) of previously untreated patients were analysed for microvessel density by staining with factor VIII (von Willebrand factor) antibody and for expression of vascular endothelial growth factor (VEGF) using an anti-VEGF-antibody. Both proteins were determined by immunohistochemistry and the expression was compared with the resistance to doxorubicin measured in vitro. Microvessel density was significantly reduced in resistant tumours when compared with sensitive tumours. Of the 98 tumours with low microvessel density 83 (85%) were resistant; whereas of the 54 tumours with high microvessel density only 34 (63%) were resistant (p = 0.004). Expression of VEGF was significantly lower in resistant than in sensitive lung carcinomas. Of the 102 tumors with low expression of VEGF, 87 (85%) were resistant; whereas of the 50 tumors with high expression only 30 (60%) were resistant (p = 0.0009). Corresponding results were obtained when the analysis was restricted to squamous cell lung carcinomas or adenocarcinomas of the lung. Analysis of microvessel density or VEGF expression and clinical data (stage, histology, metastasis) revealed no significant interrelationships. These data show clearly that poor microvessel density (vascularisation) and reduced expression of VEGF are linked with resistance to doxorubicin.