Interleukin-13 (IL-13) is a cytokine secreted by Th2 lymphocytes that is capable of inducing expression of 15-lipoxygenase (15-LO) in primary human monocytes. We recently demonstrated that induction of 15-LO requires the activation of Jak2 and Tyk2 kinases and Stats 1, 3, 5, and 6. Since IL-13-induced 15-LO expression was inhibited by H7 (a serine-threonine kinase inhibitor), we predicted that Stat serine phosphorylation may also be crucial for 15-LO expression. In this study, we present evidence indicating that IL-13-induced 15-LO mRNA expression was detectable as early as 1 h by real-time reverse transcription-PCR. We found that IL-13 induced a time-dependent serine phosphorylation of both Stat1 and Stat3, detectable at 15 min after IL-13 treatment. In addition, the activation of p38 mitogen-activated protein kinase (MAPK) was detected in a time-dependent fashion, with peak phosphorylation at 15 min after IL-13 treatment. SB202190, a p38 MAPK-specific inhibitor, markedly inhibited IL-13-induced Stat1 and Stat3 serine phosphorylation as well as DNA binding. Furthermore, treatment of cells with Stat1 or Stat3 decoys significantly impaired IL-13-induced 15-LO expression. Taken together, our results provide the first evidence that IL-13 induces p38 MAPK phosphorylation/activation, which regulates Stat1 and Stat3 serine 727 phosphorylation. Both of these events are important steps in IL-13-induced 15-LO expression in human monocytes.