Struggling to cut costs in a raging recession, companies dumped a near-record 25 million square feet of office space in the first quarter, driving vacancy up and rents down, according to data to be released today by Reis Inc.

Businesses that needed to lease space took advantage of the market weakness to extract concessions out of landlords. But the trends exacerbated financial woes for owners, especially those who owe more on their mortgages than their properties are worth.

The...