Depressed heart rate variation has been shown to predict a high mortality rate in patients with severe congestive heart failure. To determine whether the degree of altered heart rate correlates with the clinical state, 24-h Holter monitoring was performed in 21 patients (mean ejection fraction: 18 +/- 11%) at baseline and after 6 months of oral therapy. At baseline, the overall 24-h heart rate variation and night/day heart rate ratio was reduced, depending on the NYHA functional class. The typical morphology of R-R interval histograms was a sensitive marker of the clinical state at baseline: the higher the NYHA class, the smaller the R-R interval variability and standard deviation of R-R intervals (total variability NYHA III versus II: P less than 0.05). Clinical deterioration after 6 months (n = 8) was accompanied by a tendency to further shortening of the mean total R-R interval variability (676 +/- 34 to 586 +/- 25 ms). This was shown in three patients, who were reclassified to NYHA class IV. In stable patients (n = 5) and those with clinical improvement (n = 8) no significant change in R-R variability was observed. It is concluded that variations in R-R interval histogram shapes correspond to different NYHA functional classes. While severe clinical disease progression may be associated with further reductions in the heart rate variability, improvement in the clinical state of congestive heart failure is not necessarily associated with changes in heart rate behaviour.