BACKGROUND: Heart rate variability (HRV) differs by the type and stage of the disease, by age, by environment, and so on. Criteria for normal, moderately changed and pathologic values of HRV lack for routine clinical work with different groups of cardiac patients. METHODS: Twelve time and frequency domain HRV variables were analysed from 24-h Holter ECG monitoring in 2578 consecutive patients of both sexes (57% male) aged 15-91 (mean 54 +/- 13) years. Fifty-nine per cent of the patients were hospital out-patients, 15% came from the cardiac unit of an internal medicine department and 26% were undergoing stationary cardiac rehabilitation. Arrhythmias of aetiology other than coronary artery disease predominate in out-patients and coronary artery disease in in-patients. Limits of moderately and pathologically changed values were determined according to biostatistical principles. RESULTS: HRV differed by the age, sex and type of patients (MANOVA P < 0.001). Heart rate and all HRV measures decreased with age. Females had a higher heart rate but more pronounced vagally modulated activity than males. Out-patients had better HRV than in-patients. The cut-off level for a pathologic standard deviation of the normal R-R interval (a measure of overall HRV) ranged from 75 ms in female out-patients to 44 ms in male patients from the cardiac unit. Different cut-points for 12 HRV measures were obtained for 12 sub-groups of patients. CONCLUSION: In view of the HRV differences between different cardiac patients, different limits of normal and decreased HRV are proposed for daily practice.