Forearm deformity secondary to giant solitary ulna exostosis is rare. We describe a rare presentation of symptomatic solitary giant exostosis involving the entire distal ulna resulting in ulnar bowing of the forearm in a five-year-old boy. The tumour was completely resected and the defect was reconstructed with an allograft wrapped with a free autogenous periosteal tubular sleeve to deliver fresh pluripotential cells for better incorporation and integration. The distal ulna physes was preserved. An osteotomy was performed on the radius to correct the deformity. One year after surgery, the deformity remains corrected with normal bone length and excellent hand function. There is no evidence of local recurrence and the allograft has fully incorporated.