Harrington had slipped to No. 6 last week. Paul Casey, who had replaced him in the fifth spot, is back in sixth this week.
Tiger Woods is No. 1 for the 234th consecutive week and the 576th week of his career.
Woods said Monday he won't participate this week in a tournament he hosts, the Chevron World Challenge in California, because of injuries he received in a recent car accident.
Phil Mickelson is No. 2, Steve Stricker No. 3 and Lee Westwood No. 4 in the rankings.
Rounding out the Top 10, in order, are Henrik Stenson, Sergio Garcia, Kenny Perry and Rory McIlroy.
Copyright 2009 by United Press International.