The $78m (£50m) cost of a performance stock option scheme for managers pushed Willis into loss for the second quarter in 2002, despite strong revenue and earnings growth.
The scheme, which started in the third quarter of last year, rewards managers for meeting or beating 2001 and 2002 targets.
The reward has not yet been paid, but was accounted for in the quarter's figures because management expects to meet all the necessary targets.
So far, about 76% of the total cost has been accounted for, based on the company's current stock price.
The accounting charge pushed Willis into a net loss for the quarter that ended on 30 June of $7m (£4.5m), compared with a net income of $17m (£10.8m) for the same quarter last year, but the company's business performance beat analysts' expectations.
Reported revenues increased 22% in the second quarter to $411m (£261m) and operating cash earnings, which excludes the cost of the stock option scheme and other variables, increased by 136% to $59m (£37.5m), compared to $25m (£15.9m) a year ago.
The company's stock was up 3.9% in New York on Tuesday when the results were announced.