The present study investigated how the RR interval complexity and variability and their circadian rhythms alter for patients with congestive heart failure (CHF). Sixteen patients aged between 41 and 72 years with CHF and 20 control subjects were included. 24-h ambulatory electrocardiographic recordings were analyzed, and digitized data was partitioned into sections of 30-min duration. For each section, time- and frequency-domain indices, and complexity indices of heart rate variability were calculated. For CHF patients, 24-h average values of all indices were significantly decreased. The circadian rhythms of mean RR intervals were preserved and resembled the abnormal circadian rhythms of the low-frequency power. The circadian rhythms of high-frequency power and all complexity indices shown in the normal control were lost. Conclusively, the patients with CHF showed decreased RR interval complexity and loss of its circadian rhythm, in addition to decreased frequency-domain RR interval variability and its abnormal circadian rhythm.