Brachial plexus birth palsy resolves spontaneously in a majority of patients, however, others may have serious permanent dysfunction. Although nerve transfers or grafts are early options for treatment, many children have residual deficits or present too late for such procedures. In these patients, rotational osteotomy of the humerus may restore improved function and motion. Unfortunately, traditional humeral osteotomies only provide correction in a single plane, therefore appropriate correction of the typical residual deformity is incomplete. Here, we describe a novel technique for obtaining a calculated correction in 3 planes using a single osteotomy of the humerus on the basis of a mathematical equation. Nine patients are described here with an average of 35.4 months follow-up. Corrections were obtained in adduction, extension, and either internal or external rotation depending on the initial deformity and Modified Mallet scores were collected for each patient. There was 1 case of transient radial nerve palsy with no long-term complications overall.