The case is reported of an infant who had a large vascular malformation involving his left arm and axilla. It was initially believed to be purely lymphatic in composition but some venous elements were identified subsequently, at operation. The lesion was unusual in that there was a total absence of skin over one area of it at birth, that it underwent spontaneous shrinkage in the early weeks of life, and that a circumferential scarring developed which led to severe functional disability of the limb. At 12 months of age the patient developed a profound cyclic thrombocytopenia that spontaneously resolved after 1 year. The cause of the platelet cycling is unresolved but might have been secondary to intermittent production by the malformation of a cytokine which was destructive against the platelets.