On November 30, 2001, about 1225 eastern standard time, a McDonnell Douglas DC-9-82, N488AA, operated as American Airlines flight 321 from New York to Chicago, piloted by an airline transport rated captain and copilot, sustained a serious in-flight injury to one passenger when the flight encountered turbulence near London, Ontario, Canada. The flight diverted to Detroit Metropolitan Wayne County Airport (DTW), near Romulus, Michigan. The 14 CFR Part 121 scheduled domestic passenger flight was operating on an IFR flight plan. Visual meteorological conditions prevailed at the time of the accident. The 2 flight crewmembers, 3 cabin crewmembers, and remaining 84 passengers were uninjured. The flight originated from La Guardia Airport, near New York, New York, at 1102, and was enroute to Chicago O'Hare International Airport (ORD), Chicago, Illinois at the time of the accident. The flight landed at DTW at 1247. The passenger stated: When I got up to go to bathroom there was no warning of any turbulence, there was no seat belt sign. Outside weather, lighting conditions I recall as normal. I saw one person waiting for front end bathroom. As such I went to back of the plane. As I was going I passed 1 flight attendent collecting cups and another sitting near the entrance of toilet. She mentioned to me some one is in the toilet. As she said that the person in came out of the toilet. I do not recall any voice announcement of turbulence while I was in the toilet. I went to wash my hands and it took less than a minute total in the toilet. As I was coming out, may be two feet from the entrance of the toilet I was thrown up-down a number of times. I tried to sit or catch the seats in vain. I saw two flight attendants seated and every attempt I made to catch something was failed. It was extremely severe turbulence. ... I fell on the ground. I saw my rt ankle is completely dislocated. Once the turbulance was over two flight attendants helped me to be on the seats. The captain stated: ... Seatbelt sign was on from beginning of flight due to light chop. Ride began to deteriorate at [flight level] FL280 so we descended to FL240. Ride was still light chop until a point past [London VOR] YXU30 on [Jet airway] J36. We encountered a pocket of severe turb[ulence]. Immediately requested descent to FL200. After receiving clearance to descend ride smoothed out passing FL200. Flight attendants called up on the interphone and reported a man who was exiting aft lav as we hit the turb had injured his foot. Flight attendants reported to captain that foot was possibly broken. Crew notified [Air Traffic Control] ATC of medical emergency and requested landing at DTW and for medical assistance to meet aircraft. First officer flew aircraft and coordinated with ATC while captain called dispatch and coordinated with flight attendants. Uneventful landing was made. Paramedics came up aft airstair at the gate and assisted injured passenger. A flight attendant stated: ... I told each person who got up to use the rest room that, the seat belt sign was on and, that they should be in their seats with their seat belts on and, that, they should not be up. During the whole flight it was choppy. The Captain made a PA after take off around 15-20 minutes into the flight that he was going to leave the seat belt sign on because it was choppy and we had just come in from ORD and that flight was bumpy. 'So for their safety' he said, 'he was going to leave the sign on until he felt it was safe enough to turn it off.' 'So he said, please stay in your seat with your seat belts on.' After I cleaned up the galley I went out to pick up. [The flight attendant] (#4) was up front. I came to the back and decided to take a seat in row 31D. It was still bumpy. [The #4 flight attendant] was then on her way back and she sat down in row 30-Seat F. The cabin was clear of passengers and so was the bathrooms. Then [the], (passenger) got up and came to the back. I told him that 'the seat belt sign was on and that he should be in his seat with his seat belt on.' He continues to the RH bathroom. [The #4 flight attendant] got up and a PA from the aft PA. She told the passengers that, 'the seat belt sign was on and they should remain in their seat with their seat belts on.' She then returned to row 30 seat F. About 1-2 minutes after her PA the passenger ... came out of the lav. It was bumpier then so I told him to take a seat. Right now in row 30-seat D. As he started in for row 30 we hit severe turbulence. Both myself and F/A (#4) tried to grab him. He flew up and down in the air about 4 to 5 times until the turbulence stopped.