Since 1987, >10,000 individuals worldwide have received immunizations with human immunodeficiency virus (HIV) preventive vaccine constructs. Many constructs elicit antibodies detected by standard serologic tests (enzyme immunoassays, rapid tests, and Western blots) and result in vaccine recipients' serum being identified as reactive and indicative of HIV infection. To determine the frequency of vaccine-induced HIV antibody among uninfected HIV vaccine trial participants and to identify factors associated with these results, serum samples from HIV-uninfected participants from selected United States phase I/II HIV-1 vaccine trials were tested with 6 serologic screening tests. Reactive specimens were tested by use of Western blot. Overall, 490 serum specimens from 461 vaccine recipients were tested; 100 (20.4%) reacted on at least 1 serologic test, and 65 (13%) were determined to be positive by Western blot. Canarypox or vaccinia vaccine recipients' serum with or without HIV envelope glycoprotein (gp120 or gp160) boosts accounted for all positive Western blot results; no positive Western blot results were obtained from gp120 subunit recipients. The potential for vaccine recipients being misclassified as HIV infected increased with vaccine complexity.