IL-10 is an anti-inflammatory cytokine which may modulate disease expression in RA. Three dimorphic polymorphisms within the IL-10 gene promoter have recently been identified and appear to influence regulation of its expression. The 1082*A allele has been associated with low and the 1082*G allele with high in vitro IL-10 production. We have analysed 117 unrelated Caucasoid RA patients and 119 ethnically matched controls. No significant differences in the allele frequencies of the three polymorphisms were found between controls and RA patients. In contrast, a significant association between the 1082*A allele and the (-1082*A/-819*C/-592*C) haplotype and IgA RF+ve/IgG RF-ve patients was observed. The association of genotypes encoding low IL-10 production with IgA RF in RA is incompatible with its suggested role in antibody isotype switching. IgA RF has been associated with severe RA and may thus be indirectly correlated with a genotype encoding low IL-10 production.