Hydoxyurea induces senescence-like growth arrest in normal human fibroblasts. p21(WAF/CIP1/SDI1), a cyclin dependent kinase inhibitor, was found to be upregulated during this growth arrest. Levels of p21(WAF/CIP1/SDI1) protein and mRNA were increased nine-fold by hydroxyurea in these cells. In order to determine whether p21(WAF/CIP1/SDI1) mRNA is increased by hydroxyurea at the transcriptional level, human fibroblast cells were transfected with reporter constructs containing a p21(WAF/CIP1/SDI1) promoter fragment and then treated with hydroxyurea. The luciferase activities in the reporter-transfected fibroblast cells were not increased by hydroxyurea, indicating that p21(WAF/CIP1/SDI1) transcription was not elevated by hydroxyurea. The half-life of the p21(WAF/CIP1/SDI1) mRNA was increased by 2.5-fold but that of p21(WAF/CIP1/SDI1) protein was not. Our results suggest that increased mRNA stability is the major mechanism of p21(WAF/CIP1/SDI1) elevation in the hydroxyurea-induced growth arrest of human fibroblasts.