Tumour necrosis factor (TNF) plays a pivotal role in the pathogenesis of rheumatoid arthritis (RA) and it has been shown that the TNF-lymphotoxin (TNF-LT) region influences susceptibility to RA. To investigate the role of the TNF-LT locus further, inheritance of TNF 5' promoter alleles was determined in multiplex RA families. Six previously defined TNF promoter single nucleotide polymorphisms (SNPs) (-233 -308, -376, -857 -863 -1031) were observed in these families and in addition. a heretofore undocumented adenine (A) to cytosine (C) substitution at position -572 relative to the transcription start site was defined. TNF 5' promoter SNPs were found to cc-segregate with specific TNF microsatellite haplotypes. In particular, the SNP -308A allele was found to be inherited with the TNF a2, b3, c1, d1, e3 (H2) microsatellite haplotype (P < 0.001) which had previously been found to be associated with RA in individuals heterozygous for the HLA-DR 'shared epitope' (SE). When the data were stratified by the presence of the SE with further stratification according to SE DR subtypes and analysed by transmission disequilibrium test (TDT) for which offspring were assumed independent, the -303A and -857T alleles were found to be associated with RA in patients carrying the SE (P = 0.0076 and 0.0063 respectively). The data were further stratified to analyse for association in individuals homozygous or heterozygous for SE alleles. Results showed that the -308A allele was significantly associated with RA susceptibility in individuals heterozygous for the SE (P < 0.001) with the significance only occurring in patients carrying HLA-DR4 (P < 0.001), while the -857T allele was significant in individuals homozygous for the SE (P = 0.0039). Further analysis using the pedigree disequilibrium test (PDT) which conservatively adjusts for all sources of familial correlation except that conferred by linkage disequilibrium still indicated a significant role for the -308A and -857T alleles. These data provide evidence that TNF promoter SNPs may play an independent role in RA susceptibility in specific immunogenetically-defined groups of RA patients.