Signal Transducers and Activators of Transcription (STAT) 5A/B regulate cytokine-inducible genes upon binding to GAS motifs. It is not known what percentage of genes with GAS motifs bind to and are regulated by STAT5. Moreover, it is not clear whether genome-wide STAT5 binding is modulated by its concentration. To clarify these issues we established genome-wide STAT5 binding upon growth hormone (GH) stimulation of wild-type (WT) mouse embryonic fibroblasts (MEFs) and MEFs overexpressing STAT5A more than 20-fold. Upon GH stimulation, 23 827 and 111 939 STAT5A binding sites were detected in WT and STAT5A overexpressing MEFs, respectively. 13 278 and 71 561 peaks contained at least one GAS motif. 1586 and 8613 binding sites were located within 2.5 kb of promoter sequences, respectively. Stringent filtering revealed 78 genes in which the promoter/upstream region (-10 kb to +0.5 kb) was recognized by STAT5 both in WT and STAT5 overexpressing MEFs and 347 genes that bound STAT5 only in overexpressing cells. Genome-wide expression analyses identified that the majority of STAT5-bound genes was not under GH control. Up to 40% of STAT5-bound genes were not expressed. For the first time we demonstrate the magnitude of opportunistic genomic STAT5 binding that does not translate into transcriptional activation of neighboring genes.