Although the growth of the proximal epiphysis of the humerus is thought to contribute 80% of the total increase in humeral length, few articles have provided evidence for this. A 9-year-old boy with a pathological fracture at the humeral neck, owing to a solitary bone cyst, was treated by the decompression method using cannulated screws for 7.5 years. During this period, we measured the longitudinal humeral bone growth. The value obtained for the longitudinal growth contribution of the proximal humerus was 88% in this particular case.