Attachment disorganization in infancy is a risk factor for behavior problems and other psychopathology. Traditionally the role of parental behavior for qualitative differences in early attachment relationships has been emphasized. However, disrupted infant-parent interactions only partly explain attachment disorganization. A complementary focus on child factors such as early differences in the underlying neurobiological systems is needed. We examined whether early structural differences in the gangliothalamic ovoid, comprising the basal ganglia and the thalamus, are involved in the etiology of infant attachment disorganization. Gangliothalamic ovoid diameter was measured by ultrasound in 6-week-old participants of a prospective population-based cohort study. Attachment classification of 629 of these infants was assessed with the strange situation at 14 months of age. Neurobiological differences within the normal range were prospectively associated with attachment disorganization. Infants with a larger gangliothalamic ovoid at 6 weeks had a lower risk of attachment disorganization at 14 months (OR = 0.73 per SD increase in diameter, 95% CI 0.57-0.93, p < .05). Volume of the lateral ventricles as an index of general brain development was not associated with attachment disorganization. These findings provide new insight into the etiology of infant attachment disorganization that may in part be neurodevelopmentally determined.