Thirty one white patients from 14 families with multiple cases of rheumatoid arthritis (RA) and 42 of their healthy relatives were completely HLA typed. In contrast with class I antigens, the class II antigens DR1 and DR4 were significantly more common in the patients than in a group of 200 healthy local white controls (DR1: 32% v 12%; DR4: 48% v 28%, in patients and controls respectively). Owing to the small number of cases the data from this study were combined with those of published reports. Examination of patients for DR1 and DR4 homozygosity and DR1/4 heterozygosity showed an increase of DR1 homozygous patients, which was not statistically significant. There was no striking deviation from random expectation in haplotype sharing of affected sib pairs. These results are compatible with a dominant influence of DR1 and DR4 in the mode of inheritance. The nearly random haplotype sharing and the molecular relation between DR1 and DR4 support the hypothesis of a direct influence of these antigens in the pathogenesis of RA. Only 68% of the patients in this study possessed either DR1 or DR4, possibly indicating a subtype of RA which is independent of HLA. Clinical and serological variables were measured and indicated no significant difference between DR1 (or DR4) positive and DR1 (or DR4) negative disease. In this small group of patients the clinical course of RA seemed to be determined mainly by other genetic or environmental factors.