Introduction: Acute pancreatitis is an inflammatory disease with a diverse aetiology and variable clinical course. The IL-l gene cluster has been implicated in this disease. Aims: The aims of the study were to investigate polymorphisms of the genes encoded within the IL-l gene cluster in patients with acute pancreatitis and normal controls and to determine the relationship between the polymorphisms and protein levels. Methods: Genotype and allele frequencies were determined in controls (n=217) and patients with acute pancreatitis (n=137) using the polymerase chain reaction (PCR) followed by digestion with restriction endonucleases where applicable. Protein levels were determined using in vitro stimulation of PBMCs followed by Enzyme Linked Immunosorbent Assay (ELISA). Patients were categorised according to severity, organ failure scores and aetiology. Results: Allele l of the VNTR86 polymorphism in the IL-l RN gene was significantly increased in the severe group of patients compared to controls (81.9% vs 63.0%, x2=9.38, p=0.002, Pc=0.004) and in the idiopathic group compared to controls (82.4% vs 63.0%, x2=9.33, p=0.002, Pc=0.004). The polymorphisms within the genes and between the genes were strongly linked. Significantly more of the Mspl-VLP-VNTR86-Sspl 2-3-2-2 haplotype was observed in the control (15.7% vs 0.1%, x2 =2528.11, p<0.000000l, Pc<0.0000001) and patient (14.0% VS 0.1%, x2 =4368.10, p<0.000000l, Pc<0.0000004) populations than expected. Significantly more of the Pstl-Aval-Alul-Taql 2-2-2-1 haplotype was observed in controls (27.7% vs 9.7%, x2 =31.39, p<0.000000l, Pc<0.0000005) and patients (12.5% vs 2.0%, x2=53.69, p<0.000000l, Pc=0.0000007) than expected. Preferential combinations of the genotypes existed within controls and patients. The median IL-lα and IL-lβ protein levels from unstimulated PBMCs were significantly increased in patients compared to controls: median values (interquartile range). In the IL-lα study, significant differences were found at 24 hours: 193.5 (127.5-363.5) pg/ml vs 1.0 (0.0-3.0) pg/ml, p=0.005, 48 hours: 256.5 (171.5-417.0) pg/ml vs 6.5 (2.0-16.0) pg/ml, p=0.006 and at 72 hours: 210.5 (138-427) pg/ml vs 0.5 (0-7) pg/ml, p=0.005. In the IL-l β study, significant differences were found at 24 hours: 663 (507-782) pg/ml vs 12 (5-53) pg/ml, p=0.004, 48 hours: 620 (570-1080) pg/ml vs 14.5 (11-36) pg/ml, p=0.004 and at 72 hours: 545.5 (442-771) pg/ml vs 12.5 (2-43) pg/ml, p=0.006. Conclusion: Polymorphisms of the IL-l gene cluster are associated with susceptibility to and/or severity of the acute pancreatitis. Polymorphisms within the IL-l gene cluster are in linkage disequilibrium. Unstimulated PBMCs from patients with acute pancreatitis secrete significantly more IL-la and IL-IP protein levels compared to those from controls. The (AC)n, Alu I and VNTR86 polymorphisms do not correspond to differences in functional protein levels.