Introduction of new diagnostic criteria and the development of HLA-typing and microbiological--serological techniques have aided in classifying and differentiating inflammatory rheumatic diseases. Reviewing epidemiological prevalence studies shows that inflammatory rheumatic diseases are often so called benign polyarthritides and that arthritides, classified as definite and probable rheumatoid arthritis (RA), may belong to a group of benign or undifferentiated arthritides. Own data show that only 25% of patients of an early synovitis out-patient clinic have definite or probable RA, 25% have an undifferentiated arthritis. The term "undifferentiated" refers to the heterogeneity of diseases included. We conclude that few epidemiologic studies exist concerning undifferentiated arthritis and spondarthritis and combined efforts are needed to distinguish RA from the multitude of other well characterized or yet unknown arthritides.