Using 24-hour Holter monitoring and time domain and power spectral measurements, we evaluated the variability of the heart rate and its circadian rhythm in 55 male patients with Duchenne-type progressive muscular dystrophy (DMD) to characterize their autonomic function versus findings in 20 normal controls. Comparisons were also made in patients with mild, moderate, and severe stages of DMD. The percent difference between successive RR intervals that exceeded 50 ms, a measure of parasympathetic tone, was significantly lower even in patients with early stage of DMD than in controls (p < 0.01). This trend became marked with disease progression. Power in the high-frequency (HF) range (0.15 to 0.40 Hz), a measure of parasympathetic tone, was lower (p < 0.01), and the ratio of the power in the low-frequency (LF) range (0.04 to 0.15 Hz) and that of HF range (LF/HF ratio), a measure of sympathetic tone, was higher in DMD patients versus controls (p < 0.01). This trend was also marked with disease progression. Patients with mild or moderate disease had a slight circadian alteration in HF and LF/HF ratio. Patients with severe disease had virtually no circadian rhythm in HF. Their LF/HF ratio was higher at night (p < 0.01), lower in the morning (p < 0.01), and still lower during the day (p < 0.01), the opposite of control findings. The autonomic abnormalities in DMD were thus characterized by a significant increase in sympathetic activity and a significant decrease in parasympathetic activity. Thus, heart rate variability and circadian rhythm were useful in assessing autonomic dysfunction in DMD.