Systemic autoinflammatory diseases are driven by abnormal activation of innate immunity. Herein we describe a new disease caused by high-penetrance heterozygous germline mutations in TNFAIP3, which encodes the NF-κB regulatory protein A20, in six unrelated families with early-onset systemic inflammation. The disorder resembles Behçet's disease, which is typically considered a polygenic disorder with onset in early adulthood. A20 is a potent inhibitor of the NF-κB signaling pathway. Mutant, truncated A20 proteins are likely to act through haploinsufficiency because they do not exert a dominant-negative effect in overexpression experiments. Patient-derived cells show increased degradation of IκBα and nuclear translocation of the NF-κB p65 subunit together with increased expression of NF-κB–mediated proinflammatory cytokines. A20 restricts NF-κB signals via its deubiquitinase activity. In cells expressing mutant A20 protein, there is defective removal of Lys63-linked ubiquitin from TRAF6, NEMO and RIP1 after stimulation with tumor necrosis factor (TNF). NF-κB–dependent proinflammatory cytokines are potential therapeutic targets for the patients with this disease.