MGIC Investment Corp., the largest mortgage insurer in the nation, lost $280 million in the fourth quarter, its tenth straight quarterly loss.

In the same period a year earlier, the Milwaukee-based firm lost slightly less, $275.6 million. At Dec. 31, the percentage of loans it guarantees that were delinquent, excluding bulk loans, was 15.46% compared with 9.51% a year ago.

Curt S. Culver, chairman and chief executive, said in a statement that the weak economy, higher levels of unemployment and lower home prices have led to an increase in the delinquent inventory and elevated incurred losses.

But there was some good news: MGIC has seen a sequential decline in the number of new notices received and its book of business written since implementing tighter underwriting guidelines in 2008 has improved the credit risk profile of its insurance-in-force.

Total revenue fell 1% to $405.5 million in the quarter, but beat analysts' view of $393.8 million. Despite the bad news, its stock was up as much as 14% on Tuesday.

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