The patellar retinaculum is important in patellar tracking. However, little attention has been paid to the biomechanics of the retinaculum. This study assessed the change in tension of the lateral patellofemoral ligament before and after anteromedial tibial tubercle transfer for patellar instability. Twenty-seven knees in 21 patients were studied and the mean age of the patients at surgery was 19 years. A buckle transducer was clamped on the lateral patellofemoral ligament and the transducer was calibrated to zero with the knee at full extension. The knee then was flexed 30 degrees, 60 degrees, 90 degrees, and 120 degrees, and the transducer recorded changes in tension within the lateral patellofemoral ligament. After anteromedial tibial tubercle transfer, lateral patellofemoral ligament tension was measured in the same manner. Although average lateral patellofemoral ligament tension increased with knee flexion with the maximum at 120 degrees flexion, these changes showed four different patterns measuring variability of patellar instability. An increase of lateral patellofemoral ligament tension after anteromedial tibial tubercle transfer meant that lateral retinacular release should be done concurrently to balance patellar tracking. This information may be useful in deciding the appropriate surgical procedure for each patient.