On January 1, 1995, at 1150 central standard time, a Boeing 737- 3A4, N326SW, experienced a partial loss of roll control during approach near Houston, Texas. There was no airplane damage and none of the 5 crewmembers or 61 passengers were injured. The airplane was operated by Southwest Airlines as Flight 293, scheduled domestic Part 121 service from Dallas, Texas, to the Houston, Texas, Intercontinental Airport. The airplane was level at 6,000 feet on a standard instrument arrival in visual meteorological conditions decelerating through about 240 knots. The copilot was flying the airplane when the flight encountered wake turbulence from a MD-80. Flight 293 was in a clean configuration and power reduced in order to decelerate. The autopilot was engaged in altitude hold and control wheel steering. After encountering the disturbed air the airplane began to roll right 20 degrees and the copilot responded with left yoke. The airplane then rolled into a 30 degree left bank and the copilot applied full right aileron, but the roll continued to the left. The captain took control of the airplane and applied right rudder as he maintained the aileron input. Control was regained and the remainder of the flight was normal. Inspection and evaluation of the flight control systems failed to reveal any anomalies.