To determine if vaccination induces replication of human immunodeficiency virus type 1 (HIV-1), 42 HIV-1-infected subjects with CD4 cell counts of 200-500 cells/microL were randomized to receive influenza vaccine or saline placebo. Infectious cell-associated and plasma HIV-1 RNA virus load were measured twice at baseline and then at 7, 10, 14, and 30 days after injection by quantitative microculture and branched DNA amplification. The ratios of the geometric mean plasma HIV-1 load of the four follow-up visits compared with baseline in vaccine (n = 28) and placebo (n = 14) recipients were similar (1.05 [95% confidence interval, 0.80-1.37] for vaccine; 0.96 [95% confidence interval, 0.68-1.33] for placebo; P = .90). The geometric mean ratios of plasma virus load at each follow-up visit to baseline did not differ significantly from 1.0 for each group. Infectious cell-associated virus load measures yielded similar results. CD4 cell counts declined similarly in both groups at 6 months. Influenza vaccination did not increase HIV-1 load in this controlled clinical trial.