Reduced respiratory sinus arrhythmia, measured as heart rate variability, is a reliable indicator of autonomic nervous dysfunction, reflecting a damage in vagal cardiac control. The authors studied the heart rate variability (HRV) of nine children treated for acute lymphoblastic leukemia during the different phases of cytostatic treatment utilizing heart rate processing techniques with a computer. The indices of HRV as well as the spectral components of heart rate were examined with special relation to vincristine administration. The heart rate variability was significantly reduced during the vincristine induction phases as compared to the consolidation and maintenance phases without vincristine administration. In particular, the respiratory components of the HRV during deep breathing tests were significantly reduced during vincristine treatment. The authors conclude that the measurement of the HRV is a suitable method for monitoring transient autonomic neuropathy, which these results show to be a frequent complication of vincristine treatment.