Spontaneous cutaneous wounds occur in avian embryos (chick, duck, quail) in various prominent parts of the body, notably the elbow, the knee and the outer face of feather buds. The frequency and size and the light and electron microscopic morphology of elbow wounds in the chick embryo are described. The cutaneous lesion appears in over 80% of the embryos at around 7 days of incubation, persists through 14 days, and finally heals completely at around 16 days of incubation. No trace of the wound is visible after that age. Wound healing of these spontaneous lesions was analysed with light microscopy (using indirect immunofluorescence for the localization of type I collagen, fibronectin and laminin) and electron microscopy. The main feature of the very slow healing process, as compared with the rapid cicatrization of experimental excision wounds, appears to be a continuous damage of the healing epidermis, until, finally, definitive wound closure occurs between 14 and 16 days of incubation. In the damaged region, where the epidermis is absent, the dermis exhibits an increased density of type I collagen fibres and of fibronectin. The upper face of the bare dermis is deprived of laminin. Spontaneous lesions do not occur in isolated wings explanted on the chick chorioallantoic membrane, where the wings do not become mobile and are not in contact with the amnion. The observations and explantation experiments suggest that the skin damage is caused by friction and abrasion of the bending elbow against the amnion or the amniotic fluid.