The King Charles had changed drastically in the late 17th century, when it was interbred with flat-nosed breeds. Until the 1920s, the Cavalier shared the same history as the smaller King Charles Spaniel. Breeders attempted to recreate what they considered to be the original configuration of the breed, a dog resembling Charles II's King Charles Spaniel of the Restoration.
Various health issues affect this particular breed, most notably mitral valve disease, which leads to heart failure. This will appear in most Cavaliers at some point in their lives and is the most common cause of death. The breed may also suffer from Syringomyelia, a malformation of the skull that reduces the space available for the brain. Cavaliers are also affected by ear problems, a common health problem among spaniels of various types, and they can suffer from such other general maladies as hip dysplasia, which are common across many types of dog breeds.
During the 16th century, a small type of spaniel was popular among the nobility in England. The people of the time believed that these dogs could keep fleas away, and some even believed that they could prevent forms of stomach illnesses. These dogs were sometimes called the "Spaniel Gentle" or "Comforter", as ladies taking a carriage ride would take a spaniel on their laps to keep them warm during the winter. Charles I kept a spaniel named Rogue while residing at Carisbrooke Castle; however, it is with Charles II that this breed is closely associated and it was said of him that "His Majesty was seldom seen without his little dogs". There is a myth that he even issued an edict that no spaniels of this type could be denied entry to any public place.
During the reign of King William III and Queen Mary II, the long nosed style of spaniel went out of fashion. The Pug was the favoured dog at the time in the Netherlands, and with William's Dutch origin, they became popular in England too. At this time interbreeding may have occurred with the Pug, or other flat nosed breeds, as the King Charles took on some Pug-like characteristics, but in any event the modern King Charles Spaniel emerged. In The Dog in 1852, Youatt was critical of the change in the breed:
The King Charles's breed of the present day is materially altered for the worse. The muzzle is almost as short, and the forehead as ugly and prominent, as the veriest bull-dog. The eye is increased to double its former size, and has an expression of stupidity with which the character of the dog too accurately corresponds. Still there is the long ear, and the silky coat, and the beautiful color of the hair, and for these the dealers do not scruple to ask twenty, thirty, and even fifty guineas.
During the early part of the 18th century, John Churchill, 1st Duke of Marlborough, kept red and white King Charles type spaniels for hunting. The duke recorded that they were able to keep up with a trotting horse. His estate was named Blenheim in honor of his victory at the Battle of Blenheim. Because of this influence, the red and white variety of the King Charles Spaniel and thus the Cavalier King Charles Spaniel became known as the Blenheim.
Attempts were made to recreate the original King Charles Spaniel as early as the turn of the 20th century, using the now extinct Toy Trawler Spaniels. These attempts were documented by Judith Blunt-Lytton, 16th Baroness Wentworth, in the book "Toy Dogs and Their Ancestors Including the History And Management of Toy Spaniels, Pekingese, Japanese and Pomeranians" published under the name of the "Hon. Mrs. Neville Lytton" in 1911.