BACKGROUND: Some reports have highlighted the relationship between heart rate variability (HRV) and the degree of postoperative pain experienced. This study retrospectively examined whether differences in heart rate variability may be related to the appearance of postoperative pain in patients undergoing breast cancer surgery. FINDINGS: We retrospectively analyzed 20 postoperative patients who had no pain immediately upon admission to the post-anesthesia care unit (PACU), divided into two groups: group A (n = 16) had no pain on admission to PACU, remaining pain free upon discharge (12 h after surgery); group B (n = 4) comprised patients with no pain on admission to PACU but who experienced increasing pain requiring intervention in PACU 1 h after surgery. HRV was measured immediately on admission to PACU and 2 h after surgery; this included variables of low-frequency power (LF), high-frequency power (HF), and LF/HF. There were significant differences in HF and LF/HF in group A compared with those in group B on admission to PACU (immediately after arrival): HF, group A, 35.4 +/- 18.1; group B, 64.2 +/- 9.5*; LF/HF group A, 2.7 +/- 2.4; group B, 0.6 +/- 0.2*, *p < 0.05). There was no significant difference in the Numerical rating scale (NRS) between the two groups immediately after admission to PACU. At 1 h after the surgery, NRS in Group B increased, and there were significant differences in NRS values between the two groups 1 h after surgery prior to the use of analgesic agents (NRS, group A, 1.0 +/- 0.9; group B, 4.0 +/- 1.4*, *p < 0.01). Patients in group A required no analgesic agents for at least 12 h after surgery. CONCLUSIONS: Lower HF and higher LF/HF values immediately after arrival in PACU were observed in patients remaining pain free for 12 h after surgery compared to patients who experienced increasing postoperative pain 1 h after surgery. The data suggest that differences in HRV may be related to the appearance of postoperative pain.