There are two at 21 letters each, but looks like gastroenteroanastomosis wins at 23 letters:
23 gastroenteroanastomosis
21 gastroenterocolostomy
21 glossolabiopharyngeal
The longest word starting with E is Electroencephalographically.
Floccinaucinihilipilification is the longest word starting with F; it means the act of estimating something as worthless.
Pneumonoultramicroscopicsilicovolcanoconiosis (also happens to be the longest word in the English language)
Extraterritoriality is the longest word in the English language beginning with "ex."
The longest word starting with a "P" is pneumonoultramicroscopicsilicovolcanoconiosis, which has 45 letters and is in the Guinness Book of Records. It was, however, coined for the specific purpose of becoming the longest word, and is not a specifically recognized form of silicosis.
iufijifidafhudfhafa
The longest word starting with E is Electroencephalographically.
The longest ones I could find in Webster's Second International dictionary are 17 letters long: antiagglutinating autoagglutinating
Floccinaucinihilipilification is the longest word starting with F; it means the act of estimating something as worthless.
Pneumonoultramicroscopicsilicovolcanoconiosis (also happens to be the longest word in the English language)
Extraterritoriality is the longest word in the English language beginning with "ex."
Methionylthreonylthreonylglutaminylalanyl...isoleucine. It has 189819 letters and the longest word in the world.
Another word starting with g is gloominess
The longest word starting with a "P" is pneumonoultramicroscopicsilicovolcanoconiosis, which has 45 letters and is in the Guinness Book of Records. It was, however, coined for the specific purpose of becoming the longest word, and is not a specifically recognized form of silicosis.
forethoughtfulness
Immunoelectrophoresis-- 21 letters
enormous