Vanguard's

$5.6 billion

(VHCOX) - Get Report

Capital Opportunity fund, one of the hotter mid-cap growth funds, is closing to new investors today.

The tech-heavy fund, run by subadviser

Primecap Management

, is up 157% over the past year and has a 50.9% average annual return over the past three years, which both top about three-quarters of the fund's peers, according to

Morningstar

.

TST Recommends

Predictably, the team's success and Vanguard's trusted label led to a cash geyser. Back in September, Vanguard tried to stem the tide by raising the fund's minimum investment from $3,000 to $25,000, even for IRA investments, which often have low required minimums.

But first-quarter inflows could top all of last year's intake. Through inflows and appreciation, the fund is $1 billion bigger than it was at the end of last month. The average mid-cap growth fund has just over $800 million, according to Morningstar.

The fund's managers appear to be having trouble putting all the cash to work. At year-end, the fund's cash position topped 17%. It was just over 13% at the end of last month, according to Vanguard's Web site.

Calling this a "cooling off" period, a company statement says Vanguard has no plans to reopen the fund for at least six months. Until it does reopen, the fund's more than 92,000 shareholders will still be able to invest up to $25,000 per year in the fund.

Best known as the king of index funds, Vanguard has closed hot funds in the past due to fat inflows. Most recently it closed its

(VGHCX) - Get Report

Health Care in February 1999 and

reopened it last December when it believed investors were less rabid for the fund's shares. This closing and an eventual reopening could follow the same pattern.

The Capital Opportunity fund stumbled out of the gate from 1995 to 1997. But the Primecap team took over in February 1998 and has dazzled, focusing on fast growers selling for what the team believes are modest prices. The fund's average price-to-earnings ratio is below the

S&P 500

index's, according to Morningstar.