This study evaluated the radiological and clinical outcomes of bone grafts using fourth and fifth extensor compartmental arteries (4+5 ECAs) for the treatment of Kienböck's disease. In total, 21 patients (12 men and 9 women; mean age, 41 years; range, 19-59 years) were followed for a mean of 33 months. Radiological images were analyzed for the Lichtman stage, carpal height ratio, radioscaphoid angle, and Stahl's index. Clinical evaluation included range of motion, visual analog scale (VAS) score, grip strength, modified Mayo wrist score (MMWS), Lichtman outcome score, and Disabilities of the Arm, Shoulder and Hand (DASH) score. At the time of surgery, 6, 14, and 1 patients had Lichtman stages II, IIIA, and IIIB, respectively. At the final follow-up visit, grip strength had improved from 65.4% to 79.7%, wrist extension had improved from 43° to 57°, and flexion had improved from 42° to 50°. There were no significant changes in the carpal height ratio, Stahl's index, or radioscaphoid angle. The mean VAS score was 1.7, and the mean DASH score was 6.9. The mean MMWS was 87.9, with excellent and good outcomes in 6 and 11 patients, respectively. Satisfactory Lichtman outcome scores were observed in 81%. Body mass index had a strong correlation and age had a weak correlation with MMWS (coefficient=-0.534, P=.013, and coefficient=-0.393, P=.078, respectively). The 4+5 ECA bone graft is effective for the treatment of Kienböck's disease in young patients with low body mass index. [Orthopedics. 202x;4x(x)xx-xx.].