The objective of this study was to examine whether levels of endotoxin and other markers of immuno-inflammatory activation are altered in adult patients with severe autism. We determined circulating serum endotoxin levels, its soluble receptor (sCD14), and markers of immuno-inflammatory activation (IL-1beta, IL-6, and IL-10) in 22 adult patients with severe autism and 28 age- and gender-matched healthy controls. Compared with healthy subjects, serum levels of endotoxin were significantly higher in autistic patients and inversely and independently correlated with Socialization scores on the Vineland Adaptive Behavior Scales (VABS) and ADI-R Domain A score (social). Whether increased endotoxin may contribute to the pathophysiology of inflammation and impaired reciprocal social interaction in autism should be further explored in future studies.