Rio de Janeiro-based Amil ranks as Brazil's largest healthcare company, providing health and dental benefits, hospital and clinical services and advanced care management to more than 5 million people.

The deal is expected to be completed in two steps, with UnitedHealth expected in the fourth quarter to purchase about 60% of Amil's shares outstanding pending regulatory approval, and then buy a further 30% from public shareholders in 2013.

Amil founder and CEO Edson Bueno and partner Dulce Pugliese, owners of 70% of the company, will retain a 10% stake for at least five years. Bueno has also committed to investing about $470 million in UnitedHealth shares, to be held for five years. Bueno is expected to remain as CEO of Amil, and join UnitedHealth's board of directors.

The purchase includes about $600 million in Brazilian tax benefits, making the effective equity purchase price $4.3 billion.

UnitedHealth, of Minnetonka, Minn., said that Brazil ranks only behind the U.S. as the second-largest and fastest-growing private healthcare market in the Western Hemisphere.

"Brazil has emerged as a consistently growing and evolving market for private sector health benefits and services," UnitedHealth CEO Stephen J. Hemsley said in a statement. "Combining Amil, the clear market leader serving an under-penetrated market of nearly 200 million people, with UnitedHealth Group's experiences and capabilities developed over the last three decades is the most compelling growth and value creation opportunity we have seen in years."

Amil is both an insurer and a provider of services. The company will generate revenue of about $5 billion in 2012, and owns a network of 22 hospitals and 50 clinics, while managing a care network that includes 44,000 doctors, 3,300 hospitals, 11,000 outpatient clinics and another 12,000 ancillary services facilities.

UnitedHealth said the deal would be slightly accretive to its 2013 earnings per share. The company also said it expects third-quarter earnings, to be released Oct. 16, to be at least $1.45 per share, besting the consensus analyst estimate of $1.25 per share.

A Sullivan & Cromwell LLP team led by Keith A. Pagnani and including Sergio J. Galvis, Ronald E. Creamer Jr., Theodore Edelman, Matthew M. Friestedt and William G. Farrar advised UnitedHealth.

Alberto Luzarraga of Linklaters LLP and Carlos Barbosa Mello, Gustavo Lian Haddad and Sergio Machado of LeFosse Advogados advised Amil. José Olympio Pereira and Fabio Mourao from Credit Suisse Group provided the target with financial advice.

-- David Marcus contributed to this report.