Mutations in the NALP3/CIAS1/cryopyrin gene are linked to three autoinflammatory disorders: Muckle-Wells syndrome, familial cold autoinflammatory syndrome, and chronic infantile neurologic cutaneous and articular syndrome. NALP3, with the adaptor molecule ASC, has been proposed to form a caspase-1-activating "inflammasome," a complex with pro-IL1β-processing activity. Here, we demonstrate the effect of NALP3 deficiency on caspase-1 function. NALP3 was essential for the ATP-driven activation of caspase-1 in lipopolysaccharide-stimulated macrophages and for the efficient secretion of the caspase-1-dependent cytokines IL-1α, IL-1β, and IL-18. IL-1β has been shown to play a key role in contact hypersensitivity; we show that ASC- and NALP3-deficient mice also demonstrate an impaired contact hypersensitivity response to the hapten trinitrophenylchloride. NALP3, however, was not required for caspase-1 activation by Salmonella typhimurium, and NALP3 deficiency only partially protects mice from the lethal effects of endotoxin. These data suggest that NALP3 plays a specific role in the caspase-1 activation pathway.