Objective: To investigate cortical association tracts using diffusion tensor imaging (DTI) in children with global developmental delay of unknown etiology. Study design: We performed DTI in 20 patients (age range: 18-83 months, mean: 45 ± 16 months, 12 males) with a history of global developmental delay and 10 typically developing children (age range: 26-99 months, mean: 54 ± 24 months, 5 males). DTI tractography was performed to isolate major cortical association tracts. Results: In 9 out of 20 patients, arcuate fasciculus (AF) was absent bilaterally and in another 2 patients, it was absent in left hemisphere. In contrast, AF was present bilaterally in all typically developing children. Fractional Anisotropy (FA) of inferior longitudinal fasciculus (ILF) was asymmetric in the control group but not in the developmental delay group (P = .04). FA was significantly reduced in right ILF in developmentally delayed children compared with controls (P = .03). FA of other association tracts was not different between patients and controls (P = NS). The apparent diffusion coefficient (ADC) showed no asymmetry for these tracts in controls or developmentally delayed children (P = NS). Conclusions: DTI can be used to identify absence of AF and inadequate maturation of ILF in children with global developmental delay of unknown etiology.