What factors distinguish probands from multicase rheumatoid arthritis same sex sibships from sporadic disease?

Thirty-two of 243 same sex sibships in which the proband had rheumatoid arthritis (RA) had at least one same sex sibling with RA ("familial" RA) and 211 did not ("sporadic" RA). The most important factors accounting for familial RA were the sibship size, and the proportion of siblings sharing both HLA haplotypes with the proband. Demographic and clinical details, autoantibodies and HLA-DR status were similar between the 2 groups, with the exceptions of regular use of artificial teardrops, and involvement of distal interphalangeal joints, which were both more prevalent in familial RA. The extrapolation of results from familial to sporadic RA appears to be justified.