Overlapping syndrome (OS) is usually used as the term of the combinations of three connective tissue diseases, i.e., systemic lupus erythematosus (SLE), progressive systemic sclerosis (PSS) and polymyositis (PM) or dermatomyositis (DM). OS is sometimes confused with mixed connective tissue disease (MCTD) since the definitions of the both diseases have not been established yet. Rheumatoid arthritis (RA) is a distinct disease and only exceptionally associated with the other CTD. These rare cases include destructive arthritis of SLE and PSS, multiple peripheral type of psoriatic arthritis, and arthritis associated with X-linked hypogammaglobulinemia and selective IgA deficiency. The conditions complicated with RA are not uncommon. They are osteoporosis, Sjogren's syndrome, amyloidosis and so on. There are some rare conditions or diseases which will be able to develop to RA. These peculiar cases include juvenile rheumatoid arthritis, adult onset Still's disease, polymyalgia rheumatica and palindromic rheumatism.