On July 22, 1998, at 1345 central daylight time (cdt), a Boeing 767-223, N335AA, operated by American Airlines as flight #1758, encountered moderate turbulence while climbing through flight level 180 over Peotone, Illinois. The airplane was not damaged. One passenger received serious injuries and one flight attendant received minor injuries. Two cockpit crewmembers, 5 flight attendants and 128 passengers were not injured. The 14 CFR Part 121 flight had departed Chicago's O'Hare International Airport (ORD) with an original destination of Raleigh-Durham, North Carolina. The flight returned to ORD and landed at 1406 cdt following the turbulence encounter. The airplane was in IFR conditions when the encounter occurred. The flight crew reported that while climbing through FL180, approximately 10 mile west of the Peotone VOR, they encountered moderate turbulence for a time period of about 5 seconds. They reported that they were in the clouds, but their weather radar was not showing any returns and there were no reports of turbulence either by air traffic control or by other airplanes in the area. The flight crew reported the seat belt sign was on. Shortly after encountering the turbulence they were informed that the #2 flight attendant and one passenger were injured as a result of the turbulence. The decision was made to return to the O'Hare International Airport. The injured flight attendant reported she was in the rear galley setting up for service when the turbulence was encountered. She reported, "... the back of the airplane swayed harder then usual and I pushed against the counter tops. My first thought was to get to my jumpseat but by the time I could move, we dropped and I flew up and hit my head on the ceiling. I came back down and hit the back side of my right thigh." She stated she eventually got back into her jumpseat. She recalled seeing a deadheading flight attendant helping a female passenger up off the floor. The deadheading flight attendant then provided assistance to her. Both the #2 flight attendant and the injured passenger were transported to the hospital upon landing at O'Hare. The flight attendant was treated and released from the hospital with bruises. The passenger suffered a broken rib. The deadheading flight attendant reported the injured passenger who was in seat 38H, did have her seatbelt fastened, but that the belt "...released below the seat cushion." A maintenance inspection of the seat belt revealed the male portion of the belt disconnected from the attach point. The attach point and the latching mechanism were both intact. A review of the Flight Data Recorder data revealed the airplane experienced vertical accelerations ranging from -0.14g's to 1.429g's during the turbulence encounter.