Envision Healthcare Holdings Inc. sold 20% more shares than planned in its initial public offering as the deal priced at the high end of its expectations, a strong showing for one of the last debuts before Wall Street's late-summer lull.

The medical-services company agreed to sell 42 million shares for $23 each, people familiar with the matter said Tuesday, raising $966 million before the potential sale of additional shares to underwriters. Envision had offered 35 million shares, expecting them to price at $20 to $23 apiece.

The IPO is the largest in the U.S. since building materials maker HD Supply Holdings Inc.'s $1.1 billion debut in late June. It comes just ahead of a late-August lull for deals as money managers leave for summer vacations. Since 2005, just two U.S.-listed IPOs have priced during the last two weeks of August, according to Ipreo, a capital markets data and advisory firm.

Greenwood Village, Colo.-based Envision, which provides contract emergency-room staffing for hospitals and ambulance services for local governments, has been expanding by signing new contracts and acquiring small and regional physician practices that perform similar roles. In the three months ended June 30, its profit rose 22% to $9.6 million as revenue climbed 12% to $899 million.

"The growth story is good," said Tim Nelson, a senior analyst at Nuveen Asset Management, which oversees more than $120 billion. "I expect them to continue to get new contracts and I expect them to continue to be able to buy mom-and-pop practices and gain share. I don't think it's overpriced, by any means."

Investors also expect the company to benefit from the nation's health-care overhaul, which is expected to expand coverage to millions of Americans starting next year. Though it's hard to say precisely how much because of uncertainty about the implementation of the new rules, Mr. Nelson said. In the latest quarter, Envision set aside a $791 million provision for uncompensated care.

The IPO follows big share gains by Envision's peers this year. Team Health Holdings Inc.'s (TMH) shares have gained 40% year-to-date, well ahead of the 19% gain in the S&P 500. IPC The Hospitalist Company Inc. (IPCM), a provider of physicians in hospitals and clinics, is up 35%.

Envision is returning to public ownership after a $2.9 billion buyout by private-equity firm Clayton, Dubilier & Rice LLC in February 2011. The investors also assumed $300 million in debt carried by the company, then named Emergency Medical Services Corp.

The shares are slated to start trading Wednesday under the symbol EVHC. Goldman Sachs Group Inc. is leading the deal with Barclays PLC, Bank of America Corp. and Citigroup Inc.

Write to Matt Jarzemsky at matthew.jarzemsky@dowjones.com

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