InvestMidwest, the annual venture capital forum, opened this morning with a sobering statistic: 4,000 companies in VC portfolios that are, so to speak, past their expiration date.

That number came from Mark Heesen, president of the National Venture Capital Association, who was talking about companies that received VC investments between 1991 and 2000. Most of that decade’s investments, of course, occurred during the final three years, as the dot-com bubble was inflating.

Of all investments made during that decade, about 4,000 or 35 percent are still in the VC funds’ portfolios. That’s an astonishing figure, especially given that funds were expecting quick riches when they put record amounts of money to work in 1999 and 2000. (Of the remaining investments, Heesen said, about 33 percent were acquired, 14 percent made public stock offerings and 18 percent failed.)

Typically, venture capitalists try to wind down a fund in 10 years or less. That means a lot of tough decisions have to be made in the next year or so, Heesen said:

If the acquisition market and the IPO market slow down, it makes it harder for us to deal with these companies that are still leftover from the bubble. 2008 is the year you will really start to see the impact of the bubble. That’s how long-term in nature this industry is. Things we did in 1999, 2000 and 2001 are really going to be coming home to roost in 2008.