Phosphorylation of the estrogen receptor alpha (ERalpha) N-terminal transcription activation function AF1 at serine 118 (S118) modulates its activity. We show here that human ERalpha is phosphorylated by the TFIIH cyclin-dependent kinase in a ligand-dependent manner. Furthermore, the efficient phosphorylation of S118 requires a ligand-regulated interaction of TFIIH with AF2, the activation function located in the ligand binding domain (LBD) of ERalpha. This interaction involves (1) the integrity of helix 12 of the LBD/AF2 and (2) p62 and XPD, two subunits of the core TFIIH. These findings are suggestive of a novel mechanism by which nuclear receptor activity can be regulated by ligand-dependent recruitment of modifying activities, such as kinases.