The influence of the polymorphism of the estrogen receptor-beta gene, cytosine-adenine (CA) dinucleotide repeat in intron 6, in the occurrence of rheumatoid arthritis (RA) was investigated. Forty-seven RA patients and 36 control subjects with osteoarthritis (OA) were recruited. CA repeat polymorphism was examined using denaturing high-performance liquid chromatography (WAVE DNA Fragment Analysis System). The mean number of CA repeats was significantly higher in RA than in OA patients. Two groups were established: <or=21 repeats (short S), >or=22 repeats (long L); and 3 kinds of genotypes (SS, SL, LL) were found. In RA patients, the L allele frequency was higher (OR = 2.03; P <or= 0.05 Fisher's test) and more subjects carried the LL genotype (OR = 5.50; P <or= 0.05 Fisher's test) compared to OA patients. It was concluded that the presence of the L allele could be a risk factor for RA, although additional larger scale study is needed.