On September 8, 1993, at 1740 central daylight time, a Boeing 737-300, N350SW, operated by Southwest Airlines, Inc., as revenue flight 282, sustained an in-flight structural failure of the left outboard flap assembly, while on approach to the Lambert-St. Louis International Airport, St. Louis, Missouri. There were no injuries to the 5 crewmembers or 139 passengers aboard the airplane. The flight, conducted under 14 CFR Part 121, originated at Kansas City, Missouri. An IFR flight plan was filed, and visual meteorological conditions prevailed at the time. The airplane was on final approach to runway 12R when the controller requested a go-around for traffic separation. The first officer was the pilot flying. The first officer stated in an interview the go-around was normal until he called for flaps 1 about 500 feet. He said the airplane had a rolling tendency to the left, and required a lot of right aileron. The cockpit crew received notification from the cabin attendants that there was a problem with the flaps on the left side. The Captain sent a Southwest Airlines Captain who was riding in the cockpit observers seat to the cabin to investigate. He reported pieces of the outboard flap were hanging loose. The Captain took control of the airplane and extended the flaps to 5. He made an uneventful landing and taxied to the gate. Inspection of the airplane was conducted by Federal Aviation Administration Inspectors who reported the left outboard foreflap was broken into two pieces and had damaged two sections of spoiler. The two forward bolts which hold the inboard sequencing carriage to the foreflap attach lug were fractured, and the two aft bolts were missing their nuts. The left outboard flap assembly was removed and sent to the NTSB Materials Laboratory for examination. A copy of the report of the examination of the flap assembly is appended.