Deficits in visual processing are early cognitive abnormalities in patients with Huntington's disease (HD) and may be found in presymptomatic gene carriers. We investigated the nature and evolution of deficits in visual processing in HD, and whether subtle deficits could be recognized by formal testing in asymptomatic carriers. We studied 35 patients with HD in stages 1-3 of functional disability, and 26 symptom-free relatives at 50% risk for the disease. We administered the Mini Mental State Examination to assess overall cognitive function and tests to assess visuospatial skills such as visual attention and ocular scanning (Cancellation Task and Line Bisection Test), visuoconstructive abilities (Copy of Rey's Complex Figure), and visuoperception (Hooper Visual Organization Test). The group at risk comprised 15 asymptomatic carriers (AC) and 11 non-carriers (NC) and was assessed by investigators blinded to gene status. HD patients were impaired in most of the tasks compared with AC and NC, and the scores declined steadily from stage 1 to 3. However, the difference between patients in stage 1 of HD and AC and NC in most of the tasks was not significant. Only the Hooper Test, which requires complex visual integration, was highly discriminative of early symptomatic from asymptomatic carriers (P < 0.05). There were no significant differences between AC and NC in any of the tasks. We conclude deficits in visual processing develop with other manifestations of the disease and are not significant on formal testing at presymptomatic stages; also, early visual deficits in HD seem to be related to disorders in complex visual processing.