Through December 1994, 41 healthcare workers with a documented seroconversion to human immunodeficiency virus (HIV) in temporal association to an occupational exposure were reported to the Centers for Disease Control and Prevention (CDC). Each tested positive for HIV antibodies within 12 months of the occupational exposure. Two (5%) of the 41 tested negative for HIV antibodies >6 months following the occupational exposure but were seropositive within 12 months of the injury. Both denied any subsequent exposures to HIV after the initial exposure, and in one case genetic sequencing confirmed the source of the infection. Four of the healthcare workers took postexposure zidovudine prophylaxis; each reported an acute retroviral syndrome within 6 weeks of their exposure, and each of the four seroconverted to HIV within 6 months of the exposure. Our data suggest that zidovudine prophylaxis does not delay the development of HIV antibodies beyond 6 months. Because many of the healthcare workers had follow-up testing at irregular intervals, with long periods between tests, it was not possible to define precisely when seroconversion occurred. However, our findings are compatible with previously published estimates that 95% of infected persons will develop HIV antibodies within 6 months of infection.