We wanted to investigate whether rheumatoid arthritis (RA) patients, defined by the American College of Rheumatology (ACR) 1987 criteria and selected from one community by the help of the official Finnish data registers, share the common HLA susceptibility genes. The HLA frequencies of 88 RA patients representing 85% of the prevalent cases of RA in the community were compared with those of 188 healthy controls. Fifty-four per cent of the index cases with RA had DR4 compared with 30% of the healthy controls (P <0.001). The 'RA susceptibility sequence' was found in 75% of the DRB1 genes in the index cases, but it did not correlate with the severity of the disease. The frequency of DR3 was not increased in RA patients but it was associated with features of severe disease, that is, with a high erythrocyte sedimentation rate (P<0.05), extra-articular disease (P<0.01) and prostheses in large joints (P<0.05). According to our results community-based RA patients satisfying the new ACR criteria show the common DR4 association. DR3 was the only HLA allele which showed some disease-modifying effect correlating with the severity of RA.