Visuospatial processing is accomplished in distinct neuroanatomic pathways. One such pathway, known as the where pathway, involves a dorsal route through magnocellular thalamic cells to occipital and parietal cortices and conveys location and motion information. A second pathway, known as the what pathway, involves a ventral route through parvocellular thalamic cells to occipital and temporal cortices and conveys color and form information. The where pathway is thought to be responsible for processing spatial relationships while the what pathway is responsible for object identification. Children with early-treated congenital hypothyroidism (CH) who exhibit selective visuospatial deficits may provide a good model to study the differential development of these pathways. Because children with CH lacked thyroid hormone at a time when needed by developing brain regions such as the parietal cortex, these children may be affected to a greater degree on tasks tapping where but not what pathway processing. We tested this hypothesis via retrospective analysis of their performance on 6 spatial tasks. Compared were 49 adolescents with CH and 49 matched control participants. On the basis of confirmatory factor analysis, tasks were assigned to either where or what pathway groupings. A repeated measures ANOVA showed the CH group was impaired relative to a normal comparison group only on where pathway tasks. Regression analyses indicated that severity of early hypothyroidism was the strongest predictor of where pathway processing but had no effect on what pathway tasks. It is concluded that thyroid hormone is required during late gestation and early life for the normal development of the where aspects of visuospatial processing. (JINS, 2001, 7, 556–562.)