How many sets of fathers and sons have served as Prime Minister of Great Britain?

To date, there have been two sets of father and son Prime Ministers, four men in all, William Pitt, 1st Earl of Chatham and his son, William Pitt, known as William Pitt the Younger to distinguish him from his father. The second was George Grenville and his son, William Grenville.

William Pitt served as a Whig Prime Minister from 1766 to 1768 and was the informal leader of the cabinet between 1756 and 1761. William Pitt's son, also named William served as Prime Minister spanning 3 separate terms between 1783 and 1806, for a total of 18 years. William Pitt the Younger is also noted for being the youngest person (as of 2017) to hold the office of Prime Minister, beginning his first term at the age of 24.

George Grenville served as a Whig Prime Minister between 1763 and 1765. One of his more memorable actions as Prime Minister was the enactment of the Stamp Act enforced on the American Colonies. William Grenville was Prime Minister after the death of William Pitt the Younger, from 1806 until 1807.

Pitt the Younger and William Grenville were also to date the only set of full cousins to hold the office, their fathers being brothers-in-law.