The outline of the epidemiological study of rheumatoid arthritis in the Western region of Jerusalem (population about 14,000, interview sample about 8,000, examination sample about 2,000), is given. The semantic difficulties, reliability of diagnostic criteria, evaluation of x‐ray pictures, validity of Rh. A. index with regard to cultural, social and ethnical background, and its special significance within the population of Israel, influence of non‐response to interview and/or examination on statistical results, and repeated examinations of a part of the sample in a remittant chronic disease like rheumatoid arthritis, are discussed. The relatively large number of the sample in this present study may render possible to overcome the many pitfalls involved.