OBJECTIVE: Heart rate variability (HRV) has been demonstrated to be a risk factor after acute myocardial infarction (AMI). In the present study serial measurement of SDNN (standard deviation of the mean of qualified NN-interval) in short intervals was used to assess HRV changes after AMI, and determine the role of these as independent risk factors compared to clinical, arrhythmic, ischemic and anamnestic variables. Measurements from a normal healthy middle-aged male population were used as reference (n = 63). METHODS: SDNN from a five-minute period during day and night-time, respectively, was examined in 103 patients 1 week (n = 54), 1 month (n = 72) and 12-16 months (n = 54) after infarction. RESULTS: Day SDNN did not change during one-and-a-half years after AMI, and was significantly reduced compared with healthy males. Night SDNN, low after 1 week, with recovery 1 month after AMI, was significantly reduced compared with healthy males early, but not late after AMI. Thus, the study indicated during day-time a continuous abnormal sympathetic preponderance in the course of 16 months after AMI, and during night-time a gradual recovery of parasympathetic preponderance beginning early after AMI. CONCLUSION: One week after AMI day-time SDNN of <30 ms, and night-time SDNN of < 18 ms, age > or =60 years, and myocardial ischemia (Holter monitoring) were independent predictors of 9 years\' mortality. One and 12-16 months after AMI reduced day and night-time SDNN had no prognostic implication.