STUDY OBJECTIVE: To investigate whether pupil diameter (PD) measured during scotopic conditions is influenced by pain in conscious patients in the early postoperative period. DESIGN: Prospective, observational, cohort study. SETTING: Single-center, postanesthesia care unit (PACU). PATIENTS: Patients scheduled for a surgery during general anesthesia. INTERVENTIONS: Baseline PD was measured the day before surgery. Patients were observed on admission to the PACU, immediately after extubation, during the different steps of analgesic intervention (demand, relief, plus intermediate measures when relevant), and either at discharge or 3 hours after admission. MEASUREMENTS: PD, pain (numerical rating scale), and alertness (Observer's Assessment of Alertness/Sedation scale). MAIN RESULTS: Of 103 patients enrolled, 80 required analgesia in the PACU and completed follow-up. Pain intensity evolved in line with expectations (temporary increase then relief), and alertness increased with time. PD increased from low mean values at admission to the PACU (40% of baseline) to a plateau throughout the rest of the study period (80% of baseline) and was not related to pain intensity. Multivariate analyses suggested that the factors influencing PD (or its value related to baseline) were time since extubation and the type of opioid (remifentanil, sufentanil, or sufentanil at high doses) administered during surgery. CONCLUSIONS: Because of a residual effect of intraoperative opioids and a level of nociceptive stimulation lower than in surgical conditions, PD is not significantly influenced by early postoperative pain or pain relief.