IL-1R-associated kinase (IRAK)-1 is a critical mediator of TLR/IL-1R-induced activation of the transcription factor NF-κB. We previously described that a commonly occurring IRAK-1 variant haplotype, containing amino acid changes from serine to phenylalanine at position 196 and from leucine to serine at position 532, is associated with increased activation of NF-κB in LPS-stimulated neutrophils from patients with sepsis-induced acute lung injury and also higher mortality and more severe clinical outcomes in such patients. To investigate the underlying molecular mechanisms, we examined the ability of wild-type and variant IRAK-1 to modulate NF-κB activation. We found increased NF-κB transcriptional activity and expression of NF-κB-dependent proinflammatory cytokines in IL-1β-stimulated IRAK-1-deficient cells transfected with variant IRAK-1 as compared with IRAK-1 wild type. IκB-α degradation was faster and p65 phosphorylation more prolonged after IL-1β stimulation in cells expressing the IRAK-1 variant. However, IL-1-induced activation of MAPKs and nuclear translocation of NF-κB are comparable in both IRAK-1 variant- and IRAK-1 wild-type-expressing cells. Autophosphorylation of the IRAK-1 variant is greater than that found with wild-type IRAK-1. Additionally, variant IRAK-1 has greater interaction with TNFR-associated factor 6 than does wild-type IRAK-1. The enhanced activity of variant IRAK-1 appeared to be due to the alteration at aa 532, with only minimal effects being associated with change at aa 196. These results demonstrate that variant IRAK-1 is associated with alterations in multiple intracellular events that are likely to contribute to increased NF-B activation and inflammatory responses in individuals with this IRAK-1 haplotype. Copyright © 2007 by The American Association of Immunologists, Inc.