The incidence, evolution and prognostic significance of ventricular arrhythmias after myocardial infarctions were studied over a 2 year period in 144 patients (108 men, 36 women: mean age 64 +/- 2 years) by Holter monitoring on the 20th day, 6th month (104 recordings), 12th month (94 recordings), 18th month (76 recordings) and 24th month (82 recordings). A total of 500 recordings was obtained, 471 of which were of good enough quality to be interpreted. On the 20th day, the prevalence of ventricular arrhythmias was independent of the site of myocardial infarction: they were classified as follows (Lown and Wolff grading): no ventricular arrhythmias (34 patients); moderate (Grade I) arrhythmias (73 patients, 50%); severe ventricular arrhythmias (Grades 2, 3, 4, 5) (37 patients, 26%). Fourty-seven patients underwent all 5 Holter recordings, the evolution of the arrhythmias could therefore be followed in detail. Despite considerable individual variation, the overall evolution indicated a clear aggravation at 6 months compared to the recording taken on the 20th day. At the 12th month, there was a higher incidence of arrhythmias but they were of a lower grade in 72% of cases: there was a steady improvement thereafter until the 24th month with the incidences falling to those observed on the 20th day. At the end of 2 years, there were 102 survivors; 41 patients died, and one was lost to follow-up. Over half the deaths (23 cases) occurred during the first 6 months, 4 during the second, 7 during the third and 7 during the fourth 6 months. The cause of death was cardiac in 21 cases, was of other causes in 4 cases and was unknown in 16 cases. The correlation between the death rate and results of Holter monitoring on the 20th day showed: only 4 of the 41 deaths had no recorded ventricular arrhythmia, there was a moderate or severe ventricular arrhythmia in 37 cases; in these patients, the cause of death (cardiac or other) was not related to the degree of arrhythmia. These results confirm previously reported preliminary studies indicating the value of Holter monitoring on the 20th day of myocardial infarction for assessing the 2 year prognosis. Death occurred in I ou of 10 Patients without ventricular arrhythmias, in 1 out of 4 patients with a moderate ventricular arrhythmia, and in 1 out of 2 patients with severe arrhythmias (p less than 0.001). The evolution was characterised by a "critical period", the first year, during which most deaths and aggravation of ventricular arrhythmias (in survivors) were observed; during the second year, there was a progressive improvement.(ABSTRACT TRUNCATED AT 400 WORDS)