Steroid receptors mediate their actions by using various coregulatory proteins. We have recently characterized ARIP3/PIASx alpha as an androgen receptor (AR)-interacting protein (ARIP) that belongs to the PIAS [protein inhibitor of activated STAT (signal transducer and activator of transcription)] protein family implicated in the inhibition of cytokine signaling. We have analyzed herein the roles that four different PIAS proteins (ARIP3/PIASx alpha, Miz1/PIASx beta, GBP/PIAS1, and PIAS3) play in the regulation of steroid receptor- or STAT-mediated transcriptional activation. All PIAS proteins are able to coactivate steroid receptor-dependent transcription but to a differential degree, depending on the receptor, the promoter, and the cell type. Miz1 and PIAS1 are more potent than ARIP3 in activating AR function on minimal promoters. With the natural probasin promoter, PIAS proteins influence AR function more divergently, in that ARIP3 represses, but Miz1 and PIAS1 activate it. Miz1 and PIAS1 possess inherent transcription activating function, whereas ARIP3 and PIAS3 are devoid of this feature. ARIP3 enhances glucocorticoid receptor-dependent transcription more efficiently than Miz1 or PIAS1, and all PIAS proteins also activate estrogen receptor- and progesterone receptor-dependent transcription but to a dissimilar degree. The same amounts of PIAS proteins that modulate steroid receptor-dependent transcription influence only marginally transactivation mediated by various STAT proteins. It remains to be established whether the PIAS proteins play a more significant physiological role in steroid receptor than in cytokine signaling.