Hypoxia-inducible factor 1 (HIF-1) is a transcription factor that regulates genes involved in adaptation to hypoxia. Expression of HIF-1α was evaluated in rat and human prostate cancer cell lines. Increased expression of HIF-1α mRNA in rat prostate cancer cell lines and hypoxia-induced expression of HIF-1α protein in human prostate cancer cell lines are associated with increased cell growth rates and metastatic potential. HIF-1α mRNA was undetectable in the normal rat ventral prostate by Northern blot hybridization. HIF-1α protein expression and HIF-1 DNA binding activity were detected in normoxic PC-3 cells. Human prostate cancer cells plated at low density manifested higher functional HIF-1α expression than cells plated at high density independent of O₂ tension. HIF-1α may become dysregulated in prostate cancer and thus drive the transcription of hypoxia-adaptive genes involved in tumor progression. This is also the first evidence that human cancer cells can express functional HIF-1α protein under normoxic conditions.