Saccadic reaction time or latency reflects the operation of the largely cortical mechanisms that decide where to look. An unusual feature of latency is that it varies randomly from trial to trial. In the past, this variation has been regarded as a nuisance, but analysis of its statistical properties has turned out to provide much insight into the underlying cerebral decision mechanisms. Thanks to recent technological improvements, large number of data can be gathered quickly and noninvasively using miniature, portable devices, offering the possibility of using saccadometry as a biomarker to enhance the diagnosis and monitoring of neurological conditions. Here we show that this technique can robustly distinguish genetically identified presymptomatic and symptomatic Huntington's disease patients from age-matched controls, largely through an increase in the number of unusually early responses, which might reflect an impairment of the tonic suppression of the colliculus normally mediated by pathways through the basal ganglia.