Delayed hypersensitivity to four common antigens was assessed in 32 patients undergoing major abdominal surgery randomly allocated to either general anesthesia (fentanyl + O2/N2O + postoperative pain relief with systemic opiates) or general anaesthesia + epidural analgesia (local anaesthetics + morphine) continued for 72 h. Skin-test responses were performed 2 days before surgery and 1 day after surgery and compared to a similar retesting schedule in 16 comparable non-operative control patients. Cumulated mean skin-test responses increased from 1290 to 2330 mm2 (P less than 0.0001) during retesting in the non-operative control group. In contrast, mean skin-test responses in patients operated during general anaesthesia + systemic opiates for postoperative pain relief fell from 1422 to 1227 mm2 (P = 0.3) and in patients receiving epidural analgesia from 1228 to 890 mm2 (P = 0.06), without statistically significant differences between these two groups (P greater than 0.5). Thus, surgery leads to depression of delayed hypersensitivity and this impairment in immunofunction is not modified by an epidural analgesic regimen providing adequate pain relief.