We postulated that the lowest instrumented vertebra (LIV) can be determined based on the apical vertebra. Seventy-two Lenke type 1 patients (average age: 13.6 years) receiving posterior spinal fusion were randomised into two groups. In group A, the apical vertebra was used to determine the LIV and in group B the neutral vertebra was used. All patients had Cobb angles <90° and average follow-up was over three years. Posteroanterior and lateral standing radiographs were used to assess flexibility, Cobb angle and distance from the C7 line to the central sacral vertical line. Both methods produced statistically significant changes in Cobb angle, sagittal T5-T12 and plumb line deviation, and there were no significant differences between the two methods. More cases of imbalance occurred in group B (five) than group A (two). We conclude that the apical vertebra can be used to determine the LIV in patients with Lenke type 1 adolescent idiopathic scoliosis.