Startle reflex studies in rodents indicate that female are more reactive than rats in experimental models of sustained anxiety but not in models of phasic fear (Toufexis, 2007). This study examined evidence for a similar effect in humans. Participants were exposed to three conditions, (1) predictable aversive shocks signaled by a cue, (2) unpredictable shocks, and (3) no shocks. Acoustic startle stimuli were delivered regularly across conditions. Phasic startle potential to the threat cue in the predictable condition was not affected by sex. In contrast, and consistent with basic research, the sustained increase in startle in the predictable and unpredictable conditions was greater in women compared to men. Animal studies suggest that such an effect may be mediated by the effects of sexual dimorphism in limbic structures, including the bed nucleus of the stria terminalis. However, psychosocial factors may also contribute to this effect. Keywords fear; anxiety; sex difference; startle; unpredictability The greater prevalence of mood and anxiety disorders among women compared to men (Kessler et al., 1994) points to a potential sex difference in reactivity to threat. Preclinical studies may provide clues as to the nature of this difference. A recent review focusing on animal models, using the startle reflex as an operational measure of aversive states, compared sex differences in two types of aversive responses, anxiety, a sustained state of distress to uncertain threat, and fear, a phasic defensive response to certain danger (Toufexis, 2007). Contrasting phasic and sustained aversive states was motivated by evidence of a neural differentiation between them, the former relying on the central nucleus of the amygdala (CeA) and the latter on the bed nucleus of the stria terminalis (BNST; Davis, 1998), as well as by an indication of strong sexual dimorphism in the BNST (Allen & Gorski, 1990). Toufexis concluded that BNST-mediated sustained potentiated startle (anxiety), but not cued specific fear-potentiated startle (fear), was increased in female rats compared to males.The present study examined evidence for a similar sex difference in humans, also using startle as a measure of aversive states. We have distinguished phasic fear to a predictable threat signaled by a cue from the more sustained contextual anxiety state during administration of predictable (certain) and unpredictable (uncertain) shocks (Grillon et al., 2006; Grillon, Baas, Lissek, Smith, & Milstein, 2004a). Based on Toufexis's review (Toufexis, 2007), we hypothesized that relative to men, women would show enhanced contextual-potentiated startle but not increased fear-potentiated startle to a threat cue.