We prospectively examined the incidence of postoperative nausea and vomiting (PONV) in a group of 193 elderly surgical inpatients receiving no postoperative antiemetic prophylaxis. Risk factors for PONV and detailed data on postoperative opioid use were recorded. The overall postoperative vomiting (POV) rate was 23.8%, whereas postoperative nausea (PON) was 51.3%. Opioid use (P = 0.025), and female gender (P = 0.038) were identified as significantly influencing POV in this relatively small population. There was a strong logarithmic dose-response relationship between postoperative opioid dose and POV (r2= 0.98, P < 0.01), as well as PON (r2= 0.98, P = 0.01). Use of patient-controlled analgesia or epidural analgesia was a marker for large-dose opioid use (P < 0.001) and was associated with POV in the 24-h postoperative period of 41% and 31% respectively, compared with 11% for other patients (P < 0.001). Future studies defining risk factors for POV should treat postoperative opioid use as a continuous variable, rather than treat it as a dichotomous variable.