Opioid withdrawal is a crucial and recurring event during the course of opioid abuse that has a negative impact on the immune system. In this study, we investigated whether abrupt withdrawal (AW) or precipitated withdrawal (PW) potentiates human immunodeficiency virus (HIV) infection of human T lymphocytes. AW and PW enhanced HIV infection of peripheral blood lymphocytes and T-cell lines (Jurkat and CEMX174). In addition, both AW and PW induced HIV replication in a latently HIV-infected human T-cell line (J1.1). The enhancing effect of AW and PW was associated with the induction of neuropeptide substance P in both peripheral blood lymphocytes and the T-cell lines. The substance P receptor antagonist, CP-96,345, not only blocked AW- or PW-induced endogenous substance P expression but also abrogated AW- or PW-induced HIV replication in T cells. These findings provide a cellular mechanism that supports the notion that opioids have a co-factor role in promoting HIV infection of the immune cells.