On February 23, 2012, about 1430 central standard time, a Southwest Airlines Boeing 737-7H4, N915WN, encountered 2 to 3 seconds of moderate turbulence while in cruise flight between Chicago, Illinois, and Los Angles, California. The Captain and the First Officer, both of whom were airline transport pilots, were not injured, nor were any of the 135 passengers. One of the three flight attendants received minor injuries, and a second flight attendant received a serious injury to her right hand. The airplane was not damaged. The 14 Code of Federal Regulations Part 121 scheduled domestic passenger flight, which departed Chicago Midway Airport about 55 minutes prior to the turbulence encounter, was en route to Los Angeles International Airport, Los Angeles, California. At the time of the event, the airplane was in visual meteorological conditions on an Instrument Flight Rules flight plan. According to the flight crew, they were in level cruise flight in smooth air at Flight Level 360, when the airplane experienced the turbulence encounter. Although the airplane had encountered periods of light chop earlier in the flight, it had been in completely smooth air for about 30 minutes prior to the event, and there had been no report of turbulence from the Air Route Traffic Control Center, nor had the flight crew overheard any other aircrew stating that there was anything but smooth air in the area of the encounter. Although the flight attendants were in the process of serving drinks, the passenger seatbelt sign was still on from the earlier period of choppy air. At the moment of the encounter, the airplane experienced both a momentary altitude deviation and a left roll of less than 10 degrees, both of which were corrected by the autopilot, with a slight overshoot past the wings level position. During the encounter sequence, one flight attendant was thrown back against the aft galley door, whereupon her hand hit the recyclable can door. She then immediately felt pain in the hand. At that point in time, the Captain came on the public address system and directed the flight attendants to take their seats immediately, which they did until the flight crew was convinced that they had returned to smooth air conditions. Although the injured flight attendant continued her in-flight duties until the plane landed in Los Angeles, she ultimately went to Urgent Care for treatment. X-rays taken of her hand determined that she had a closed fracture of the carpal bone in her right hand. According to both flight crew members, the airplane reacted in a manner consistent with it flying through the wake of another preceding or crossing airplane, and after the few seconds of the turbulence encounter, followed by the correction by the autopilot, the airplane returned to straight and level flight in smooth air.