Objective-To study the association between rheumatoid arthritis (RA) and HLA and tumour necrosis factor (TNF) polymorphism in Peruvian mestizo patients in comparison with ethnically similar controls. Methods-Seventy nine patients with RA and 65 ethnically matched healthy controls were genotyped for HLA-DRB1, HLA-DQA1, HLA-DQB1, and TNF and TNF alleles using PCR amplification. Clinical severity was assessed as mild, moderate, or severe in 35 of the patients. Results-TNF 6 showed the strongest association with disease susceptibility. The TNF 6 allele was more common in patients than in controls (p<0.0076) and the proportion of patients with at least one copy of this allele was greater (p<0.015, relative risk 2.35). Among the HLA-