At the age of eighteen Sydenham attended Magdalen Hall, Oxford; after a short period his college studies appear to have been interrupted, and he served for a time as an officer in the Parliamentarian army during the Civil War. He completed his Oxford course in 1648, graduating as bachelor of medicine, and about the same time he was elected a fellow of All Souls College. It was not until nearly thirty years later (1676) that he graduated as MD, not at Oxford, but at Pembroke Hall, Cambridge, where his eldest son was by then an undergraduate.