BACKGROUND Chemical thoracic sympathectomy (CTS) resulted in profound bradycardia in a patient with severe post-therapeutic neuralgia. To clarify the cause of this bradycardia, the authors evaluated heart rate variability using a Poincaré plot, which is a scatter diagram of the current R-R interval plotted against the R-R interval immediately preceding it, in this patient and in others scheduled for CTS or mandibular block (MB). METHODS Twenty-three patients were scheduled for CTS (n = 13, CTS group) and for MB (n = 10, MB group). Heart rate and the SD of the R-R interval variabilities spreading along the x axis (SDRR) and perpendicularly along the diagonal line of the Poincaré plot (SDdeltaRR) were evaluated before, just after, and 1 h after the block. RESULTS Neither group had significant changes in heart rate. The MB group showed no significant change in the SD(RR):SDdeltaRR ratio. In the CTS group, however, the SD(RR):SDdeltaRR ratio decreased significantly from 1.72+/-0.20 to 1.23+/-0.11 just after CTS. The previous patient, who had a high SD(RR):SDdeltaRR ratio of 3.45 before CTS, exhibited severe bradycardia (22 beats/min). CONCLUSIONS The SD(RR):SDdeltaRR ratio decreased after CTS without any significant concomitant change in heart rate. The decrease in the SD(RR):SDdeltaRR. ratio indicates a reduction of cardiac sympathetic activity. However, CTS in patients having high SD(RR):SDdeltaRR ratios can result in profound bradycardia.