The North American banking unit of the Netherlands' ABN Amro Holding said Thursday that it had an agreement to buy CNBC Bancorp for an undisclosed amount.

CNBC, which has $850 million in assets, is the holding company for Columbia National Bank of Chicago, the 20th-largest bank in the Chicago area.

Merger deals have been proliferating in the city. Chicago, long a fragmented but lucrative banking market, has recently attracted a number of midwestern banks eager to build market share.

ABN Amro North America Inc. has announced six such deals in the region since the start of 1990, including the CNBC pact, and has completed most of them. Earlier this year the company agreed to buy Comerica Inc.'s Illinois branches for $190 million.

Standard Federal Bank of Troy, Mich. and Associated Banc-Corp of Green Bay, Wis., two banks with no presence in the market, recently agreed to acquisitions there.

The Chicago merger boom exemplifies a growing trend, national banks wanting to increase market share in key markets, said bank analyst Anthony Howard. "Chicago has been a hot market for mergers the last two months, and the activity should continue," said Mr. Howard, who works for First Michigan Corp. in Detroit.

ABN Amro's LaSalle National Bank is the second largest in Cook County, with 12.6% of deposits, Mr. Howard said. The CNBC deal would probably nudge the bank's deposit share over 13%, he said. First Chicago NBD Corp. is the market leader, with 18.5%

CNBC has eight branches and 10 automatic teller machines. After the acquisition, LaSalle would have 130 branches and 180 ATMs in the Chicago area.

LaSalle can be expected to expand outside Chicago, too, Mr. Howard said. Ohio, Indiana, and Michigan could be target states.

The CNBC deal, which requires regulatory and shareholder approval, is expected to close in the fourth quarter.

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