SniffDNZ098, this question has been asked quite a few times, and rarely fails to start a few arguments (and maybe some cyber-chest-thumping). Bottom line: Are you talking about NIC fraternities and NPC sororities? NPHC GLOs? Today's numbers or last month's numbers? (They change all the time). Check out these threads (found by doing a thread-title only search of the word "largest"):
What is the largest national sorority? (Fraternities did get included in the discussion.)
Largest Fraternities and sororities
What is the largest sorority?
Oh well, at least I got this posted before anyone said that Chi Omega is the largest women's organization (in terms of numbers) except for the Girl Scouts.
Edited to correct spelling.