AbstractCo‐infections with HIV‐1 and the human T leukemia virus types 1 and 2 (HTLV‐1, HTLV‐2) occur frequently, particularly in large metropolitan areas where injection drug use is a shared mode of transmission. Recent evidence suggests that HIV‐HTLV co‐infections are associated with upregulated HTLV‐1/2 virus expression and disease. An in vitro model of HIV‐1 and HTLV‐1/2 co‐infection was utilized to determine if cell free HIV‐1 virions or recombinant HIV‐1 Tat protein (200–1,000 ng/ml) upregulated HTLV‐1/2 expression and infectivity. Exposure to HIV‐1 increased the number of HTLV‐1 antigen expressing cells, from 6% at baseline to 12% at 24 hr, and 20% at 120 hr (P < 0.05) post‐exposure. A similar, although less robust response was observed in HTLV‐2 infected cells. HIV‐1 co‐localized almost exclusively with HTLV‐1/2 positive cells. Exposure to HIV‐1 Tat protein (1,000 ng/ml) increased HTLV‐1 p19 expression almost twofold by 48 hr, and cells co‐stimulated with 10 nM phorbol myristate acetate (PMA) showed almost a fourfold increase over baseline. It is concluded that HIV‐1 augments HTLV‐1/2 infectivity in vitro. The findings also suggest a role for the HIV‐1 Tat protein and PMA‐inducible cellular factors, in HIV‐1 induced HTLV‐1/2 antigen expression. J. Med. Virol. 80:494–500, 2008. © 2008 Wiley‐Liss, Inc.