Thomas Sydenham (1624–1689) was one of the greatest early English physicians and has been called ‘the Father of English Medicine’. Sydenham’s achievement was to introduce an entirely new spirit into the medicine of the time, laying emphasis on clinical observation and accurate descriptions of disease. He was not concerned with medical theory and scorned those who were. This led to hostility between Sydenham and other physicians in the later years of his life. He revolutionised the treatment of smallpox and fevers in general and was the first person to recognise hysteria as a distinct disease. This work contributed to his fame as a physician in his own lifetime and his legacy as an early pioneer in medicine.