Tumour cells exposed to hypoxia in vitro can show increased expression of several selected genes, including the gene encoding the vascular endothelial growth factor (VEGF), suggesting that hypoxia followed by reoxygenation might promote the malignant progression of tumours. An in vitro/in vivo study was conducted to investigate whether hypoxia can increase the angiogenic potential of tumour cells through increased VEGF secretion. Four human melanoma cell lines (A-07, D-12, R-18, U-25) were included in the study. Cell cultures were exposed to hypoxia (oxygen concentration <10 p.p.m.) in vitro using the steel chamber method. Rate of VEGF secretion was measured in vitro in aerobic and hypoxic cell cultures by ELISA. Angiogenesis was assessed in vivo using the intradermal angiogenesis assay. Aliquots of cells harvested from aerobic cultures or cultures exposed to hypoxia for 24 h were inoculated intradermally in the flanks of adult female BALB/c-nu/nu mice. Tumours developed and angiogenesis was quantified by scoring the number of capillaries in the dermis oriented towards the tumours. The number of tumour-oriented capillaries did not differ significantly between tumours from hypoxic and aerobic cultures for A-07 and U-25, whereas tumours from hypoxic cultures showed a larger number of tumour-oriented capillaries than tumours from aerobic cultures for D-12 and R-18. The VEGF secretion under aerobic conditions and the absolute increase in VEGF secretion induced by hypoxia were lower for D-12 and R-18 than for A-07 and U-25, whereas the relative increase in VEGF secretion induced by hypoxia was higher for D-12 and R-18 than for A-07 and U-25. VEGF is not a limiting factor in the angiogenesis of some tumours under normoxic conditions. Hypoxia can increase the angiogenic potential of tumour cells by increasing the secretion of VEGF, but only of tumour cells showing low VEGF secretion under normoxia. Transient hypoxia might promote the malignant progression of tumours by temporarily increasing the angiogenic potential of tumour cells showing low VEGF expression under normoxic conditions.