Various authors have reported different failure rates for the cervico-humeral flap. An attempt is made to explain these occurrences in the light of recent experimental and clinical work which has increased our knowledge of the detailed blood supply of the supraclavicular fossa and shoulder region. In particular, the role of a fascio-cutaneous vessel in the supply of a proximally based shoulder flap is stressed. Division of this vessel may occur during proximal mobilisation of the cervico-humeral flap. This fundamentally alters the nature of the flap and may be the key to the varying failure rates. The implications of this for fascio-cutaneous flaps in general are discussed.