Objective. To assess whether genetic variation in the interleukin-1 (IL-1) gene cluster contributes to familial osteoarthritis (OA) by influencing innate ex vivo production of IL-1 beta or IL-1 receptor antagonist (IL-1Ra). Methods. Innate ex vivo IL-1 beta and IL-1Ra production upon lipopolysaccharide (LPS) stimulation of whole blood cells was measured in subjects from the Genetics, Osteoarthritis and Progression (GARP) Study, which includes sibling pairs in which at least one sibling has symptomatic OA at multiple sites. Radiographic OA (ROA) was assessed by Kellgren/Lawrence score. Subjects from the GARP Study and controls from the Rotterdam Study were genotyped for 7 single-nucleotide polymorphisms (SNPs) encompassing the IL-1 gene cluster on chromosome 2q13. Linkage disequilibrium analysis and genotype and haplotype association analysis were performed to assess the relationship between the IL-1 gene cluster SNPs, innate ex vivo cytokine production, and OA. Results. Among subjects in the GARP Study, the haplotype variable-number tandem repeat in intron 2/T+8006C/T+11100C 2/2/1 of the IL1RN gene was significantly associated with reduced innate ex vivo bioavailability of IL-1 beta upon LPS stimulation (P = 0.026) and with ROA at the highest number of joint locations. Conclusion. These results show that genetic variation at the IL-1 gene cluster is associated with lower IL-1 beta bioavailability and with OA at a large number of joint locations. The data further indicate that, among subjects with OA affecting the highest number of joints, the innate immune system may be activated, thereby obscuring possible underlying mechanisms.