Almost 90% of US companies experienced some kind of computer security attack in 2005, the Federal Bureau of Investigation (FBI) has said, with many attacks costing firms an average $24,000 (£14,200).
In a study of 2,000 businesses of varied size, the FBI said 87% of firms had seen an attack on their systems, and 20% revealed they had been attacked 20 or more times last year.
Virus or spyware infections were the most common occurrences, and a smaller number reported network port scans to gain access to systems or data sabotage.
The FBI said 33% of companies reported port scans of their systems, but the bureau said the occurrence of port scan was much more common in the field, indicating that many firms were not actually aware of some attacks on their businesses.
The report of the 2,000 businesses logged around 5,400 computer attacks companies knew about.
The survey found that 64% of attacks cost companies money to put right, with the average cost being $24,000. Cleaning up virus infections were the costliest incidents, said firms.
The total cost of incidents was $31.7m and virus attacks accounted for $11.9m of this figure.