Several studies have implicated tumor necrosis factor (TNF)-a in the pathogenesis of insulin-dependent diabetes mellitus (IDDM). In the present study we analyzed the first reported TNF-a gene polymorphism in relation to IDDM. We have made frequence analysis and tested in vitro lipopolysaccharide (LPS)induced TNF-a secretion. A significant difference in allele frequency was observed between patients and controls (p = 0.03). However, a very strong association of the uncommonTNF2 allele was observed with the HLA-B8, -DR3 alleles.The relative risk (RR) ofTNF2 was 2.2 compared to a RRof 3.1 for DR3. One reason for this difference was the identification of the TNFl allele on the otherwise strongly IDDM-associated HLA-DR3 haplotype: DQB1*0201, DQA1*0501, DRB1*0301, TNFc2, TNFB*2, TNFal, TNFb5, B18. Thus, the IDDM-associated TNF2 allele had n o DR3-independent value as a disease marker. The LPS-induced TNF-a production by human monocytes in relation to genotypes demonstrated that TNF1/2 heterozygous individuals had higher, though not statistically significantly (p = 0.08) levels than TNF1-homozygous subjects. However, this difference was rather small, unlikely to be of biological significance and based on the present material we cannot establish the functional importance of this polymorphism.