Objective. Autism spectrum disorders (ASD) have been associated with atypical cortical grey and subcortical white matter development. Neurodevelopmental theories postulate that a relation between cortical maturation and structural brain connectivity may exist. We therefore investigated the development of gyrification and white matter connectivity and their relationship in individuals with ASD and their typically developing peers. Method. T1-and diffusion-weighted images were acquired from a

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... esentative sample of 30 children and adolescents with ASD (aged 8 to 18 years), and 29 typically developing children matched for age, sex, hand preference, and socioeconomic status. The FreeSurfer suite was used to calculate cortical volume, surface area, and gyrification index. Measures of structural connectivity were estimated using probabilistic tractography and tract-based spatial statistics (TBSS). Results. Left prefrontal and parietal cortex showed a relative, age-dependent decrease in gyrification index in children and adolescents with ASD compared to typically developing controls. This result was replicated in an age and IQ-matched sample provided by the Autism Brain Imaging Data Exchange (ABIDE) initiative. Furthermore, tractography and TBSS showed a complementary pattern, where left prefrontal gyrification was negatively related to radial diffusivity in the forceps minor in participants with ASD. Conclusion. The present study builds on earlier findings of abnormal gyrification and structural connectivity in the prefrontal cortex in ASD. It provides a more comprehensive neurodevelopmental characterization of ASD, involving interdependent changes in microstructural white and cortical grey matter. The findings of related abnormal patterns of gyrification and white matter connectivity support the notion of the intertwined development of two major morphometric domains in ASD. Verbal IQ mean (SD) [range] 105 (15.4) [77-139] 112 (13.3) [80-143] .01** Performance IQ mean (SD) [range] 108 (16.3) [79-147] 109 (15) [67-137] .93 Clinical characterization ADI-R Social 18.9 (5.2) [7-27] ADI-R Verbal 15.21 (4.2) [8-23] ADI-R RRB 5.9 (2.9) [0-12] ADI-R Onset 3.2 (1.2) [1-5] ADOS Total 10.9 (3.9) [5-22] ADOS Communication 3.4 (1.5) [1-8] ADOS Social 7.5 (2.8) [2-14] ADOS Stereotype Behavior 2.3 (1.3) [0-5] Medication -n (%) None 32 (82%) APS - Non-APS 7 (18%)