Age and year of onset differences in siblings with rheumatoid arthritis.

Differences in age of disease onset and calendar year of onset were studied in 46 rheumatoid arthritis (RA) sibling pairs and in unrelated pairs of RA patients attending a hospital clinic. Mean age of onset and calendar year of onset differences in each group were similar. These results support the theory that RA is caused by an interaction of genetic and environmental factors and provide evidence against a single environmental transmissible cause.