This is a report on eight cases of a rare congenital malformation in the upper extremity, consisting of a unilateral muscular hyperplasia. In addition to the hand, all segments of the upper extremity may be affected. The hyperplasia is always unilateral, preferably on the right hand side, in combination with accessory muscles. Hereditary dependence or association with other malformations has not been observed. Six of eight patients were male. Shoulder and arm function were normal in all cases. Ulnar drift of the fingers in the metacarpophalangeal joints (six of eight patients), flexion contractures of the metacarpophalangeal joints (six of eight patients) and extension contractures of the wrist (three of eight patients) to various degrees were seen. A prominence of the second and third metacarpal head with an enlarged space between them gave the affected hands a very typical appearance (six of eight patients). Deformities and functional limitations requiring surgical treatment were present in six patients. In all cases, accessory muscles were found intraoperatively and resected. The macroscopic and microscopic appearance of the muscle specimen did not differ from normal muscular tissue. In all cases, additional procedures were necessary to improve the overall function. Nevertheless, the reconstructive efforts did not lead to an entirely normal hand function or appearance. The malformation we describe can clearly be distinguished from other malformations such as arthrogryposis multiplex congenita, Freeman-Sheldon syndrome or macrodactyly. Up to now, only two other reports were found in the literature showing characteristics similar to those in our own cases. Four similar cases were observed by Benatar. From a pathomechanical point of view, a disturbance in the muscular balance seems to cause the deformities and functional limitations. This imbalance could be related to accessory muscles which are not opposed by defined antagonists or to an unbalanced hyperplasia of normally developed musculature. Surgical intervention should begin early to prevent joint stiffness. Splinting and hand therapy should precede surgical intervention. Surgical treatment should aim to restore the muscular balance by resection of accessory and hyperplastic musculature. In some cases, muscle transpositions and joint releases may have to be performed. Postoperative splinting and intensive hand therapy are mandatory to preserve the results.