This paper describes the trends observed in the treatment of myocardial infarction within the three French MONICA registers between 1985 and 1991. Data, collected according to the WHO-MONICA Protocol, relate to the initial care of patients and the treatment they were prescribed. In this study 2974 cases are included. The time delays before hospitalization do not change in spite of an increasing use of mobile emergency care units. In all three centers there is a strong progression of thrombolysis, increasing from 12% to 41% in Strasbourg, from 21% to 47% in Toulouse and from 31% to 35% in Lille. Antiplatelet drugs rise from 11% to 76% in Strasbourg, from 39% to 85% in Toulouse and from 33% to 76% in Lille. Beta-blockers also progress in the three centers. Prescription of anticoagulants, nitrates and antiarrythmics is constant. In contrast calcium blockers are less prescribed. Duration of hospital stay is much longer in Strasbourg. In Toulouse the resort to coronary angiography (96%) and to angioplasty (51%) is much more frequent than in the two other centers. Similar trends in the three regions reflect the results of the secondary prevention trials conducted in the last years. Differences concern mainly treatments and procedures that were less studied before 1991. It will be interesting to study the trends in the following years in order to estimate the influence of the trials on the homogeneity of practice in the three centers.