Signal transducers and activator of transcription (STAT) proteins are often constitutively activated in leukocytes of HIV-1(+) individuals, which frequently show a dominant expression of a C-terminally truncated isoform of STAT5 (STAT5Δ). STAT5Δ can act as a negative regulator of human immunodeficiency virus type 1 (HIV-1) expression in both CD8-depleted primary leukocytes and chronically infected promonocytic U1 cells stimulated with granulocyte-macrophage colony-stimulating factor (GM-CSF). Activated STAT5Δ can directly bind to two consensus sequences in the HIV-1 long terminal repeat (LTR) promoter; binding impairs recruitment of RNA polymerase II (Crotti, A., Lusic, M., Lupo, R., Lievens, P. M., Liboi, E., Della Chiara, G., et al. (2007). Naturally occurring C-terminally truncated STAT5 is a negative regulator of HIV-1 expression. Blood, 109, 5380-5389). One of the STAT consensus sequences overlaps with one nuclear factor κB (NF-κB) binding site; interestingly, NF-κB1/p50 homodimers, frequently detected in monocytic cells, are negative regulators of HIV transcription. Here, we show that GM-CSF stimulation of U1 cells, while not inducing NF-κB activation, leads to STAT5Δ phosphorylation and binding to the NF-κB/STAT target sequence in the HIV LTR promoter, which already associates with p50 under unstimulated conditions. STAT5Δ was found to associate with p50, but not with RelA/p65, in both U1 cells expressing endogenous proteins and 293T cells overexpressing these factors. Furthermore, GM-CSF stimulation promoted concurrent binding of STAT5Δ and p50 at the HIV LTR promoter in U1 cells. Immunoprecipitation of chromatin from GM-CSF-stimulated U1 cells confirmed in vivo binding of p50 to the viral promoter together with STAT5Δ. Thus, cytokine-activated STAT5Δ/p50 complexes can contribute to the maintenance of HIV-1 latency in monocytic cells.