Multisensory integration (MSI), the combination of various senses to form a single integrated experience of the world, is essential to everyday life. It is now widely claimed by clinicians and researchers alike that deficits in MSI are part of the phenotype of autism, although rigorous empirical evidence for this is sparse. The current study examined MSI in children (10-13 yrs.) with autism spectrum disorders (ASD) and typically developing (TD) children through analysis of dense-channel array event related potentials (ERPs) elicited in response to visual (V), auditory (A), and somatosensory (S) stimuli delivered alone or in simultaneous combination. The task was the detection of all stimulus events in a simple reaction time (RT) paradigm. Differences between multisensory (MS) and the summed responses of unisensory stimuli (US) were used as a measure of MSI. We have developed a unique trisensory “stimulation desktop� which delivers visual, auditory, and somatosensory stimuli in nearly the same location. Results demonstrate a lack of RT facilitation in excess of probability summation for multisensory stimuli for the ASD group. Unisensory ERPs are generally similar between the TD and ASD groups, however the pattern for the multisensory comparisons differed between groups, particularly for auditory/tactile conditions such that there was less difference between summed US and MS responses in the ASD group. The TD pattern typically showed decreased early (50-80 ms) activations in MS conditions compared with summed US data. These results are consistent with the suggestion (Rubenstein and Merzenich, 2003) that an altered excitation / inhibition ratio in cortical processing may underlie some aspects of the ASD phenotype. Further, “sensory defensiveness� and other idiosyncratic behaviors in ASD may occur if an obligatory experience of perceptual sensory unity eludes an individual due to over-excitation from common sensory experience.