Widespread frontal lobe abnormalities, encompassing anatomy and function, are known to be implicated in Autistic Spectrum Disorders (ASD). The correlation between neurobiology and behaviour, however, is poorly understood in ASD. The aim of this study was to investigate frontal lobe anatomy and cognitive function in individuals with ASD, compared to control subjects. Thus, we assessed whole brain and frontal lobe parenchymal volume, and grey and white matter density differences in ASD, compared to control subjects, using high resolution T1-weighted magnetic resonance imaging (MRI). Furthermore, all subjects underwent a computerized reaction time task (RTT) for cognitive assessment. No differences in total parenchymal brain volume were observed, however, autistic individuals showed significantly smaller frontal lobe parenchymal volume (FLPV) and decreased white matter density, compared to control subjects. Error rates did not differ significantly between groups during the RTT, but ASD individuals responded significantly slower to target stimuli. Furthermore, reduced FLPV correlated positively with increased reaction time in individual with ASD. Decreased FLPV could be an indicator for abnormal brain development resulting in reduced processing speed in ASD.