Fifteen cases of isolated fracture of the first rib are submitted. The mechanism of the fracture is discussed. It proved compatible with the descriptions in the literature, caused either by direct trauma to the shoulder, a sudden violent contraction of juxtacostal muscles; or else it was a chance finding, without any history of trauma and as such interpreted as a fatigue fracture. A case of Horner's syndrome complicating a fracture of the first rib is also described.