PURPOSE: To evaluate differences in autonomic nervous system (ANS) activity associated with the development of retinopathy of prematurity. METHODS: Heart rate variability (HRV) as an indicator of ANS activity was calculated in two groups of premature infants: (1) a treatment group of infants who developed type 1 ROP and underwent treatment and (2) a control group of infants who did not develop ROP more severe than stage 1 and who were matched to the treatment group in terms of age, weight, and similar risk factors, including similar frequency of intraventricular hemorrhage, bronchopulmonary dysplasia, and sepsis. HRV was analyzed during the first 5 days of life, within 5 days of initial ROP examination, and within 5 days of ROP treatment for the treatment group or, for controls, on the day of last electrocardiogram data prior to discharge. Calculations were performed for the high frequency, low frequency, and low frequency-high frequency values of the HRV components for all infants. RESULTS: Between the initial ophthalmologic evaluation and the final evaluation, there was a tendency for reduction in both the low- and high-frequency components of the HRV indices in the treatment group, whereas there was a tendency for an increase in both components of the HRV indices in the control group. The difference in the rate of change of the high frequency between groups was statistically significant (P = 0.021). CONCLUSIONS: Disruption in ANS activity may play an important role in the development and severity of ROP.