The uniformity of physicians' diagnostic language on death certificates involving rheumatoid arthritis (RA), and its influence on the medical classification and on the mortality statistics was investigated. The point of departure was a disagreement between physicians' reporting of RA on death certificates and the Swedish National Central Bureau of Statistics' (NCBS) registration of this condition, to the effect that the NCBS registered a threefold mortality increase for women and a twofold increase for men attributed to RA between 1971 and 1975, whereas the physicians had reported practically no change at all for women and a slight decrease for men between the years. It is investigated in the present study to what extent this increased NCBS registration of RA as the underlying cause of death could be explained by physicians' diagnoses for underlying non-RA conditions being disregarded in favour of RA due to classification problems with the former diagnoses. All Swedish RA death certificates for the years 1971 and 1975 were studied. A total of 1224 such death certificates were identified. Physicians' diagnostic language was found to be strongly individualistic. Out of the total of 1666 diagnostic expressions 1229 (74%) had been used only once. The corresponding proportions were 76% in 1971 and 73% in 1975. Thus, there was practically no change in the linguistic individualization between these years. The 1666 expressions could be coded by 287 ICD 8 codes. One hundred and thirty-nine different expressions had been used for rheumatoid arthritis (ICD 8 code 712.3). One hundred and fifty-six (13%) of the death certificates contained diagnostic expressions encumbered with one or more of four major kinds of classification problems: (1) Expression(s) without ICD 8 code (24 certificates); (2) Expression(s) with several codes, uncertainty about which to select (45 certificates); (3) Expression(s) with several codes, one is preferable according to the ICD (39 certificates); and (4) Expression(s) which can be coded both by one specific code and by several codes (60 certificates). In one third (35%) of these cases the expression used referred to an underlying cause of death. The alternative interpretations of all these expressions differed in most cases on a three-digit level and in some cases on chapter level in the classification. Clarification of ambiguous expressions were solicited by the NCBS in only one case in 1971 and one in 1975. The NCBS' increased registration RA as the underlying cause of death between 1971 and 1975 could not be explained by problems of classification.