BACKGOUND: For rapid onset and predictable elimination half life, remifentanil may be suitable for patients undergoing cardiac surgery with unstable vital sign and decreased metabolism. We evaluated the efficacy, safety and proper dosage of intravenous patient-controlled analgesia (IV PCA) using remifentanil in patients undergoing cardiac surgery. METHODS: Forty-nine patients scheduled for cardiac surgery with sternotomy were randomly allocated to three groups. Group F had IV PCA using fentanyl with basal rate of 0.3microgram/kg/h, bolus of 0.5microgram/kg and lockout time of 15 min. Group L had remifentanil IV PCA with basal rate of 4microgram/kg/h, and group S with basal rate of 1microgram/kg/h. Both of group L and group S had setting of bolus of 0.5microgram/kg and lockout time of 5 min. In 12, 24 and 36 hours after surgery, vital sign, arterial blood gas analysis, visual analog pain scale (VAS), sedation score, and incidences of side effects were evaluated. RESULTS: The VAS score of group L was significantly low until 24 hours after surgery compared with other groups (P < 0.001). In group L, sedation score and PaCO2 in 12 hours after surgery were significantly high compared with other groups (P < 0.05), but no treatments were needed. CONCLUSIONS: Remifentanil IV PCA with basal rate of 4microgram/kg/h showed lower VAS score compared with small dosage of remifentanil and fentanyl with basal rate of 0.3microgram/kg/h. In the safety and proper dosage, further evaluations were needed.