The effect of acute coinfections on plasma human immunodeficiency virus (HIV) load and immune activation markers was evaluated. Thirty-two HIV-infected persons were prospectively enrolled; 18 had pre-illness, acute, and follow-up specimens. Plasma HIV RNA levels were determined by reverse transcriptase-polymerase chain reaction, and serum levels of activation markers, including tumor necrosis factor (TNF)-alpha, soluble (s) TNF receptors (R)-I and -II, interleukin (IL)-2, IL-6, IL-10, sIL-2R, sCD4, and sCD8, were assessed by commercial ELISAs. Median plasma HIV load increased 7. 8-fold during illness (P=.001) and decreased 1.5-fold (P=.01) during convalescence (median, 15 days). Significant virus load reductions were limited to subjects with clinical recovery. By regression analysis, changes in plasma HIV RNA were significantly associated with changes in sTNFR-I, sTNFR-II, and sIL-2R. Increased HIV replication during acute coinfections is associated with in vivo immune activation, which underscores the need to prevent and promptly treat intercurrent illnesses.