This article is about ambivalent dynamics of hope and uncertainty within neurobiological autism research. While much literature has commented on the positive hopes and expectations that surround technoscientific projects, fewer have focused on less promissory visions - and, in particular, on the presence of uncertainty and ambiguity among working scientists. This article shows how autism neuroscientists actually talk about their research in ambivalent, entangled registers of both promising hope and deflated uncertainty. The article locates the dynamic between these in an 'intermediate terrain' of autism research - in which autism is both 'present' as an epidemiological and social force, but also 'ambiguous' as a (not yet) well-defined clinical and scientific object. It argues that neuroscientists work through this terrain by drawing not only on a discourse of unalloyed hope and promise, but by entangling their research within a more complex register of 'structured ambivalence', which includes languages of uncertainty, deflation and low expectation. As well as showing the novelty of research within autism's 'intermediate terrain', this adds to a growing literature on the 'sociology of low expectations', and analyses the presence of such feelings among scientific researchers particularly. © 2014 Macmillan Publishers Ltd.