On October 22, 1993, at about 0230 Atlantic standard time, a Boeing 767-300, N649UA, flight number 990, registered to United Air Lines Inc., operating as a 14 CFR Part 121 scheduled domestic/international passenger/cargo flight, encountered moderate turbulence, while in cruise flight at flight level 350, about 450 nautical miles southeast of Miami, Florida. Visual meteorological conditions prevailed and an IFR flight plan was filed. The airplane was not damaged. The airline transport pilot- in-command, 2 airline transport first officers, 7 flight attendants, and 95 passengers were not injured. Two passengers sustained minor injuries, and one passenger sustained a serious injury. The flight originated from Rio De Janeiro International Airport about 7 hours before the accident. The flight crew stated, while the airplane was in cruise flight at flight level 350. The weather radar was on, and no weather was depicted. About 50 miles north of Joses intersection, the airplane encountered about 15 to 20 seconds of light chop which was followed by moderate turbulence lasting about 30 to 40 seconds. The seatbelt sign was turned on, and the flight attendants were contacted to check on the status of the passengers. The initial report indicated there were no injuries. About 30 minutes later, the chief purser notified the flight crew that three passengers were complaining about back and neck injuries. Company operations was notified and the flight landed without further incident. Review of available weather radar charts, revealed no weather phenomena in the immediate vicinity of flight 990 at the time of the in-flight encounter with weather.