Background: The cryopyrin-associated periodic syndromes (CAPS) are a group of inherited disorders associated with systemic auto-inflammation. CAPS result from gain-of-function mutations in NLRP3, which result in formation of an intracellular protein complex known as the NLRP3 inflammasome. This leads to overproduction of IL-1β and other pro-inflammatory signals, resulting in inflammatory symptoms. Treatments for NLRP3-related diseases are biologic agents that directly target IL-1β. We sought to determine if the orally available small molecule NLRP3 inhibitor MCC950 could inhibit IL-1β ex vivo in a cohort of patients with autoinflammatory disease. Methods: Patients were recruited to donate blood, from which PBMCs were isolated and assayed in the presence of MCC950 to determine inhibitory efficacy. Results: We found that apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC) and mature IL-1β was higher in ex vivo PBMCs from CAPS patients than healthy donors. MCC950 inhibited production of mature IL-1β in PBMC from CAPS patients with a range of mutations and blocked NLRP3 activity in an in vitro mutation reconstitution assay. Similar results were observed with PBMC from two patients with Schnitzler’s Syndrome, another auto-inflammatory disease. Conclusions: The NLRP3 inflammasome inhibitor MCC950 blocked constitutive activation of NLRP3 observed in the PBMCs of CAPS patients. This study highlights the potential utility of NLRP3 inhibition by a small molecule for rare autoinflammatory diseases that are driven by NLRP3.