Flight No. 1172 took off from Phoenix Sky Harbor International Airport around 10 a.m. It turned back about six minutes later, landed safely and returned to the gate.
The MD-80 was carrying 140 passengers and five crewmembers. No one was injured.
An airline spokesman says mechanics believe the smoke came from a small fluid leak in an auxiliary power unit. The problem was fixed and the plane took off a short time later headed for Chicago.