We assessed the effectiveness of the 3-in-1 continuous femoral block as a form of postoperative pain relief for unilateral total knee arthroplasty (TKA). Sixty patients undergoing elective unilateral TKA under subarachnoid block were randomized into three groups. Postoperative analgesia was provided with a continuous 3-in-1 femoral nerve catheter with 0.15% ropivacaine in group A, a continuous 3-in-1 femoral nerve catheter with 0.2% ropivacaine in group B, or patient controlled intravenous morphine in group C (control group). Groups A and B received patient controlled intravenous morphine pumps for rescue analgesia. Patients in each group were followed for 72 hours postoperatively. Five patients were excluded after randomization. In the remaining 55 patients there was no statistical difference in pain score between the groups. Total morphine use was highest in group C (P < 0.05). No appreciable difference could be found with sensorimotor blockade, morphine usage and satisfaction scores when comparing groups A and B. Femoral catheter dislodgement rate was 7.9%. There was no statistical difference between the groups when comparing the day of first ambulation and the time to discharge from the hospital. Satisfaction scores were higher in group A (P = 0.028) and group B (P = 0.002) compared to group C. We conclude that a continuous 3-in-1 femoral nerve block with ropivacaine 0.15% or 0.2% for elective unilateral TKA has an opioid-sparing effect.