This study sought to determine if the severity of autonomic perturbations in patients with heart failure are affected by the presence of diabetes. Decreased HRV is frequent in diabetic patients free of clinically apparent heart disease and has been invoked as a risk factor for sudden cardiac death. However, reduced HRV is also commonly present in patients with left ventricular dysfunction. The effect of diabetes on autonomic dysfunction in this setting is not known. Holter ECGs from 69 diabetic patients and 85 nondiabetic control subjects with heart failure were analyzed. The severity of autonomic dysfunction was assessed using 24-hour time- and frequency-domain HRV analysis. Prognostically important time- and frequency-domain HRV measures (SDNN, SDANN5, total power, and ultra-low frequency power) were not different between the two groups. Time- and frequency-domain parameters modulated by parasympathetic tone (pNN50, RMSSD, and HF power) were depressed to a similar degree in the diabetic and the nondiabetic groups. The low frequency power was significantly lower in diabetic patients (5.8 +/- 0.7 vs 5.3 +/- 1.0, P = 0.02). The ratio of low to high frequency power was substantially lower in the diabetic group (2.2 +/- 0.2 vs 1.4 +/- 0.2, P < 0.0001). These differences were more apparent in insulin-treated diabetics. In the presence of heart failure, HRV parameters that are most predictive of adverse outcome are similar in diabetic and nondiabetic patients. Furthermore, during increased sympathetic stimulation in the setting of heart failure, diabetes does not worsen parasympathetic withdrawal but may mitigate sympathetic activation.