Differences in certification and coding of causes of death between countries of the European Community were studied by sending sets of case histories to samples of certifying physicians. Completed certificates were coded by national coding offices and by by a WHO reference centre. Detection fractions ranged from 60% to 92% in a first study (concerning cases of chronic obstructive pulmonary disease) and from 80% to 94% in a second study (concerning cases of cancer). A detailed analysis of the findings for the Netherlands, which performed very well in both studies, reveals a substantial frequency of errors in certification (as opposed to errors in diagnosis). Comparison of national and reference centre coding suggests that the Dutch coding process is to a certain extent adapted to the frequency of these certification errors, leading to deviations from WHO coding rules. It is concluded that certification and coding practices should be studied together and that further international standardisation of coding practices will not necessarily improve the validity of national cause of death statistics.