The Marsh report said that the purchase of terrorism insurance in 2004 varied significantly, depending on a company's total insured values and location. One trend noted by Marsh is that smaller companies, those with total insured values of less than $100 million, were less likely than larger companies to purchase terrorism coverage.

Chicago-based Aon said in its annual report on global terrorism issues that the take-up rate for commercial property brokered in its Risk Services' Property Syndication Group is now 57.1 percent?"a significant increase over the 24.1 percent take-up rate of early 2003."

Aon noted in its report, "As might be expected, take-up rates were highest in perceived high-risk locations?major metropolitan areas, iconic buildings and ?trophy risks'. However, take-up rates have recently become far more consistent for risks spanning the entire U.S."

Aon also said that the take-up rate for stand-alone terrorism coverage also appears to be growing, although not nearly to the extent of TRIA.

"While brokers and insurers continue to refine the stand-alone insurance product, the increased use of captive insurers, self-insured retentions, hybrid programs and other risk-management tools have also helped make the product more attractive," Aon said in its annual report.

Take-up rates varied significantly by region, the Marsh report said. In one area of concern noted in the report, only 23 percent of companies in Houston, the center of the nation's energy industry, purchased terrorism insurance in 2004.

Approximately 53 percent of U.S. firms in the Northeast and Midwest purchased property terrorism insurance in 2004, compared with 47 percent in the South and 34 percent in the West.

Take-up rates were the highest in Boston, 69 percent; Washington, D.C., 60 percent; Chicago, 58 percent; Dallas, 57 percent; and New York, 54 percent.

Of 10 major metropolitan areas examined in the study, the rates for property terrorism insurance were highest in Washington, D.C., followed by New York and Houston.

On the West Coast, only 39 percent of Los Angeles businesses and 37 percent of those in San Francisco purchased terrorism insurance, even though rates for coverage there were significantly lower than in many other cities, said Stephen Lundin, a managing director in Marsh's property practice.

"Clearly, several factors can influence a company's decision to purchase property terrorism insurance," Mr. Lundin said. "In addition to considering the pricing of property terrorism insurance and its terms, many businesses are making decisions based upon their perception of risk."